dotfiles/gcl-info/chap-14.texi

@node Conses, Arrays, Characters, Top
@chapter Conses

@menu
* Cons Concepts::		
* Conses Dictionary::		
@end menu

@node Cons Concepts, Conses Dictionary, Conses, Conses
@section Cons Concepts

@c including concept-conses

A @i{cons}
@IGindex{cons}
 is a compound data @i{object} 
having two components called the @i{car} and the @i{cdr}.

@group
@noindent
@w{  car  cons    rplacd  }
@w{  cdr  rplaca          }

@noindent
@w{  Figure 14--1: Some defined names relating to conses.}

@end group

Depending on context, a group of connected @i{conses} can be viewed
in a variety of different ways.  A variety of operations is provided to
support each of these various views.

@menu
* Conses as Trees::		
* Conses as Lists::		
@end menu

@node Conses as Trees, Conses as Lists, Cons Concepts, Cons Concepts
@subsection Conses as Trees

A @i{tree}
@IGindex{tree}
 is a binary recursive data structure made up of
@i{conses} and @i{atoms}:
the @i{conses} are themselves also @i{trees}
(sometimes called ``subtrees'' or ``branches''), and the @i{atoms}
are terminal nodes (sometimes called @i{leaves}
@IGindex{leaves}
). 
Typically, the @i{leaves} represent data while the branches 
establish some relationship among that data.

@group
@noindent
@w{  caaaar  caddar  cdar       nsubst         }
@w{  caaadr  cadddr  cddaar     nsubst-if      }
@w{  caaar   caddr   cddadr     nsubst-if-not  }
@w{  caadar  cadr    cddar      nthcdr         }
@w{  caaddr  cdaaar  cdddar     sublis         }
@w{  caadr   cdaadr  cddddr     subst          }
@w{  caar    cdaar   cdddr      subst-if       }
@w{  cadaar  cdadar  cddr       subst-if-not   }
@w{  cadadr  cdaddr  copy-tree  tree-equal     }
@w{  cadar   cdadr   nsublis                   }

@noindent
@w{  Figure 14--2: Some defined names relating to trees.}

@end group

@menu
* General Restrictions on Parameters that must be Trees::  
@end menu

@node General Restrictions on Parameters that must be Trees,  , Conses as Trees, Conses as Trees
@subsubsection General Restrictions on Parameters that must be Trees

Except as explicitly stated otherwise,
for any @i{standardized} @i{function} that takes a @i{parameter}
that is required to be a @i{tree},
the consequences are undefined
if that @i{tree} is circular.

@node Conses as Lists,  , Conses as Trees, Cons Concepts
@subsection Conses as Lists

A @i{list}
@IGindex{list}
 is a chain of @i{conses} in which the @i{car} of each
@i{cons} is an @i{element} of the @i{list}, 
and the @i{cdr} of each @i{cons} is either the next
link in the chain or a terminating @i{atom}.  

A @i{proper list}
@IGindex{proper list}
 is a @i{list} terminated by the @i{empty list}.
The @i{empty list} is a @i{proper list}, but is not a @i{cons}.

An @i{improper list}
@IGindex{improper list}
 is a @i{list} that is not a @i{proper list};
that is, it is a @i{circular list} or a @i{dotted list}.

A @i{dotted list}
@IGindex{dotted list}
 is a @i{list} that has a terminating @i{atom}
that is not the @i{empty list}.  A @i{non-nil} @i{atom} by itself
is not considered to be a @i{list} of any kind---not even a @i{dotted list}.

A @i{circular list}
@IGindex{circular list}
 is a chain of @i{conses} that has no termination 
because some @i{cons} in the chain is the @i{cdr} of a later @i{cons}.

@group
@noindent
@w{  append      last           nbutlast  rest       }
@w{  butlast     ldiff          nconc     revappend  }
@w{  copy-alist  list           ninth     second     }
@w{  copy-list   list*          nreconc   seventh    }
@w{  eighth      list-length    nth       sixth      }
@w{  endp        make-list      nthcdr    tailp      }
@w{  fifth       member         pop       tenth      }
@w{  first       member-if      push      third      }
@w{  fourth      member-if-not  pushnew              }

@noindent
@w{  Figure 14--3: Some defined names relating to lists.}

@end group

@menu
* Lists as Association Lists::	
* Lists as Sets::		
* General Restrictions on Parameters that must be Lists::  
@end menu

@node Lists as Association Lists, Lists as Sets, Conses as Lists, Conses as Lists
@subsubsection Lists as Association Lists

An @i{association list}
@IGindex{association list}
 is a @i{list} of @i{conses} 
representing an association of @i{keys} with @i{values}, 
where the @i{car} of each @i{cons} is the @i{key} 
and the @i{cdr} is the @i{value} associated with that @i{key}.

@group
@noindent
@w{  acons  assoc-if      pairlis  rassoc-if      }
@w{  assoc  assoc-if-not  rassoc   rassoc-if-not  }

@noindent
@w{  Figure 14--4: Some defined names related to assocation lists.}

@end group

@node Lists as Sets, General Restrictions on Parameters that must be Lists, Lists as Association Lists, Conses as Lists
@subsubsection Lists as Sets

@i{Lists} are sometimes viewed as sets by considering their elements
unordered and by assuming there is no duplication of elements.

@group
@noindent
@w{  adjoin         nset-difference    set-difference    union  }
@w{  intersection   nset-exclusive-or  set-exclusive-or         }
@w{  nintersection  nunion             subsetp                  }

@noindent
@w{       Figure 14--5: Some defined names related to sets.     }

@end group

@node General Restrictions on Parameters that must be Lists,  , Lists as Sets, Conses as Lists
@subsubsection General Restrictions on Parameters that must be Lists

Except as explicitly specified otherwise,
any @i{standardized} @i{function} that takes a @i{parameter}
that is required to be a @i{list} should be prepared to signal 
an error of @i{type} @b{type-error} if the @i{value} received is a @i{dotted list}.

Except as explicitly specified otherwise,
for any @i{standardized} @i{function} that takes a @i{parameter}
that is required to be a @i{list}, 
the consequences are undefined 
if that @i{list} is @i{circular}.

@c end of including concept-conses

@node Conses Dictionary,  , Cons Concepts, Conses
@section Conses Dictionary

@c including dict-conses

@menu
* list::			
* null (System Class)::		
* cons (System Class)::		
* atom (Type)::			
* cons::			
* consp::			
* atom::			
* rplaca::			
* car::				
* copy-tree::			
* sublis::			
* subst::			
* tree-equal::			
* copy-list::			
* list::			
* list-length::			
* listp::			
* make-list::			
* push::			
* pop::				
* first::			
* nth::				
* endp::			
* null::			
* nconc::			
* append::			
* revappend::			
* butlast::			
* last::			
* ldiff::			
* nthcdr::			
* rest::			
* member::			
* mapc::			
* acons::			
* assoc::			
* copy-alist::			
* pairlis::			
* rassoc::			
* get-properties::		
* getf::			
* remf::			
* intersection::		
* adjoin::			
* pushnew::			
* set-difference::		
* set-exclusive-or::		
* subsetp::			
* union::			
@end menu

@node list, null (System Class), Conses Dictionary, Conses Dictionary
@subsection list                                                         [System Class]

@subsubheading  Class Precedence List::

@b{list},
@b{sequence},
@b{t}

@subsubheading  Description::

A @i{list}
@IGindex{list}
 is a chain of @i{conses} in which the @i{car} of each
@i{cons} is an @i{element} of the @i{list}, and the @i{cdr} of
each @i{cons} is either the next link in the chain or a terminating
@i{atom}.

A @i{proper list}
@IGindex{proper list}
 is a chain of @i{conses} terminated by 
the @i{empty list}
@IGindex{empty list}
, @t{()}, which is  itself a @i{proper list}.
A @i{dotted list}
@IGindex{dotted list}
 is a @i{list} which has a terminating @i{atom} 
that is not the @i{empty list}.
A @i{circular list}
@IGindex{circular list}
 is a chain of @i{conses} that has no termination
because some @i{cons} in the chain is the @i{cdr} of a later @i{cons}.

@i{Dotted lists} and @i{circular lists} are also @i{lists}, but usually
the unqualified term ``list'' within this specification means @i{proper list}.
Nevertheless, the @i{type} @b{list} unambiguously includes @i{dotted lists} 
and @i{circular lists}.

For each @i{element} of a @i{list} there is a @i{cons}.
The @i{empty list} has no @i{elements} and is not a @i{cons}.

The @i{types} @b{cons} and @b{null} form an @i{exhaustive partition} 
of the @i{type} @b{list}.

@subsubheading  See Also::

@ref{Left-Parenthesis},
@ref{Printing Lists and Conses}

@node null (System Class), cons (System Class), list, Conses Dictionary
@subsection null                                                         [System Class]

@subsubheading  Class Precedence List::
@b{null},
@b{symbol},
@b{list},
@b{sequence},
@b{t}

@subsubheading  Description::

The only @i{object} of @i{type} @b{null} is @b{nil}, 
which represents the @i{empty list} and can also be notated @t{()}.

@subsubheading  See Also::

@ref{Symbols as Tokens},
@ref{Left-Parenthesis},
@ref{Printing Symbols}

@node cons (System Class), atom (Type), null (System Class), Conses Dictionary
@subsection cons                                                         [System Class]

@subsubheading  Class Precedence List::
@b{cons},
@b{list},
@b{sequence},
@b{t}

@subsubheading  Description::

A @i{cons} is a compound @i{object} having two components,
called the @i{car} and @i{cdr}. These form a @i{dotted pair}.
Each component can be any @i{object}.

@subsubheading  Compound Type Specifier Kind::

Specializing.

@subsubheading  Compound Type Specifier Syntax::

(@code{cons}@{@i{@t{[}car-typespec @r{[}cdr-typespec@r{]}@t{]}}@})

@subsubheading  Compound Type Specifier Arguments::

@i{car-typespec}---a @i{type specifier},
		    or the @i{symbol} @b{*}.
  The default is the @i{symbol} @b{*}.

@i{cdr-typespec}---a @i{type specifier},
		    or the @i{symbol} @b{*}.
  The default is the @i{symbol} @b{*}.

@subsubheading  Compound Type Specifier Description::

This denotes the set of @i{conses} 
whose @i{car} is constrained to be of @i{type} @i{car-typespec} and
whose @i{cdr} is constrained to be of @i{type} @i{cdr-typespec}.
(If either @i{car-typespec} or @i{cdr-typespec} is @b{*},
 it is as if the @i{type} @b{t} had been denoted.)

@subsubheading  See Also::

@ref{Left-Parenthesis},
@ref{Printing Lists and Conses}

@node atom (Type), cons, cons (System Class), Conses Dictionary
@subsection atom                                                                 [Type]

@subsubheading  Supertypes:: 

@b{atom},
@b{t}

@subsubheading  Description::

It is equivalent to @t{(not cons)}.  

@node cons, consp, atom (Type), Conses Dictionary
@subsection cons                                                             [Function]

@code{cons}  @i{object-1 object-2} @result{}  @i{cons}

@subsubheading  Arguments and Values::

@i{object-1}---an @i{object}.

@i{object-2}---an @i{object}.

@i{cons}---a @i{cons}.

@subsubheading  Description::

Creates a @i{fresh} @i{cons}, the @i{car} of which is @i{object-1}
and the @i{cdr} of which is @i{object-2}.

@subsubheading  Examples::

@example
 (cons 1 2) @result{}  (1 . 2)
 (cons 1 nil) @result{}  (1)
 (cons nil 2) @result{}  (NIL . 2)
 (cons nil nil) @result{}  (NIL)
 (cons 1 (cons 2 (cons 3 (cons 4 nil)))) @result{}  (1 2 3 4)
 (cons 'a 'b) @result{}  (A . B)
 (cons 'a (cons 'b (cons 'c '@t{()}))) @result{}  (A B C)
 (cons 'a '(b c d)) @result{}  (A B C D)
@end example

@subsubheading  See Also::

@ref{list}

@subsubheading  Notes::
If @i{object-2} is a @i{list}, @b{cons} can be thought of as
producing a new @i{list} which is like it but has @i{object-1} prepended.

@node consp, atom, cons, Conses Dictionary
@subsection consp                                                            [Function]

@code{consp}  @i{object} @result{}  @i{generalized-boolean}

@subsubheading  Arguments and Values::

@i{object}---an @i{object}.

@i{generalized-boolean}---a @i{generalized boolean}.

@subsubheading  Description::

Returns @i{true} if @i{object} is of @i{type} @b{cons};
otherwise, returns @i{false}.

@subsubheading  Examples::
@example
 (consp nil) @result{}  @i{false}
 (consp (cons 1 2)) @result{}  @i{true}
@end example

The @i{empty list} is not a @i{cons}, so

@example
 (consp '()) @equiv{} (consp 'nil) @result{}  @i{false}
@end example

@subsubheading  See Also::

@ref{listp}

@subsubheading  Notes::

@example
 (consp @i{object}) @equiv{} (typep @i{object} 'cons) @equiv{} (not (typep @i{object} 'atom)) @equiv{} (typep @i{object} '(not atom))
@end example

@node atom, rplaca, consp, Conses Dictionary
@subsection atom                                                             [Function]

@code{atom}  @i{object} @result{}  @i{generalized-boolean}

@subsubheading  Arguments and Values::

@i{object}---an @i{object}.

@i{generalized-boolean}---a @i{generalized boolean}.

@subsubheading  Description::

Returns @i{true} if @i{object} is of @i{type} @b{atom};
otherwise, returns @i{false}.

@subsubheading  Examples::
@example
 (atom 'sss) @result{}  @i{true}
 (atom (cons 1 2)) @result{}  @i{false}
 (atom nil) @result{}  @i{true}
 (atom '()) @result{}  @i{true}
 (atom 3) @result{}  @i{true}
@end example

@subsubheading  Notes::

@example
 (atom @i{object}) @equiv{} (typep @i{object} 'atom) @equiv{} (not (consp @i{object}))
 @equiv{} (not (typep @i{object} 'cons)) @equiv{} (typep @i{object} '(not cons))
@end example

@node rplaca, car, atom, Conses Dictionary
@subsection rplaca, rplacd                                                   [Function]

@code{rplaca}  @i{cons object} @result{}  @i{cons}

@code{rplacd}  @i{cons object} @result{}  @i{cons}

@subsubheading  Pronunciation::

@b{rplaca}: pronounced ,r\=e 'plak e
	      or pronounced ,re 'plak e

@b{rplacd}: pronounced ,r\=e 'plak de
	      or pronounced ,re 'plak de
	      or pronounced ,r\=e 'plak d\=e
	      or pronounced ,re 'plak d\=e

@subsubheading  Arguments and Values::

@i{cons}---a @i{cons}.

@i{object}---an @i{object}.

@subsubheading  Description::

@b{rplaca} replaces the @i{car} of the @i{cons} with @i{object}.

@b{rplacd} replaces the @i{cdr} of the @i{cons} with @i{object}.

@subsubheading  Examples::
@example
 (defparameter *some-list* (list* 'one 'two 'three 'four)) @result{}  *some-list*
 *some-list* @result{}  (ONE TWO THREE . FOUR)
 (rplaca *some-list* 'uno) @result{}  (UNO TWO THREE . FOUR)
 *some-list* @result{}  (UNO TWO THREE . FOUR)
 (rplacd (last *some-list*) (list 'IV)) @result{}  (THREE IV)
 *some-list* @result{}  (UNO TWO THREE IV)
@end example

@subsubheading  Side Effects::

The @i{cons} is modified. 

Should signal an error of @i{type} @b{type-error}
			      if @i{cons} is not a @i{cons}.

@node car, copy-tree, rplaca, Conses Dictionary
@subsection car,    cdr,
@subheading caar,   cadr,   cdar,   cddr,
@subheading caaar,  caadr,  cadar,  caddr,  cdaar,  cdadr,  cddar,  cdddr,
@subheading caaaar, caaadr, caadar, caaddr, cadaar, cadadr, caddar, cadddr,
@subheading cdaaar, cdaadr, cdadar, cdaddr, cddaar, cddadr, cdddar, cddddr
@flushright
@i{[Accessor]}
@end flushright

@code{car}  @i{x} @result{}  @i{object}
(setf (@code{car} @i{x}) new-object)@*

@code{cdr}  @i{x} @result{}  @i{object}
(setf (@code{cdr} @i{x}) new-object)@*

@code{\vksip 5pt}  @i{x} @result{}  @i{object}
(setf (@code{\vksip 5pt} @i{x}) new-object)@*

@code{caar}  @i{x} @result{}  @i{object}
(setf (@code{caar} @i{x}) new-object)@*

@code{cadr}  @i{x} @result{}  @i{object}
(setf (@code{cadr} @i{x}) new-object)@*

@code{cdar}  @i{x} @result{}  @i{object}
(setf (@code{cdar} @i{x}) new-object)@*

@code{cddr}  @i{x} @result{}  @i{object}
(setf (@code{cddr} @i{x}) new-object)@*

@code{\vksip 5pt}  @i{x} @result{}  @i{object}
(setf (@code{\vksip 5pt} @i{x}) new-object)@*

@code{caaar}  @i{x} @result{}  @i{object}
(setf (@code{caaar} @i{x}) new-object)@*

@code{caadr}  @i{x} @result{}  @i{object}
(setf (@code{caadr} @i{x}) new-object)@*

@code{cadar}  @i{x} @result{}  @i{object}
(setf (@code{cadar} @i{x}) new-object)@*

@code{caddr}  @i{x} @result{}  @i{object}
(setf (@code{caddr} @i{x}) new-object)@*

@code{cdaar}  @i{x} @result{}  @i{object}
(setf (@code{cdaar} @i{x}) new-object)@*

@code{cdadr}  @i{x} @result{}  @i{object}
(setf (@code{cdadr} @i{x}) new-object)@*

@code{cddar}  @i{x} @result{}  @i{object}
(setf (@code{cddar} @i{x}) new-object)@*

@code{cdddr}  @i{x} @result{}  @i{object}
(setf (@code{cdddr} @i{x}) new-object)@*

@code{\vksip 5pt}  @i{x} @result{}  @i{object}
(setf (@code{\vksip 5pt} @i{x}) new-object)@*

@code{caaaar}  @i{x} @result{}  @i{object}
(setf (@code{caaaar} @i{x}) new-object)@*

@code{caaadr}  @i{x} @result{}  @i{object}
(setf (@code{caaadr} @i{x}) new-object)@*

@code{caadar}  @i{x} @result{}  @i{object}
(setf (@code{caadar} @i{x}) new-object)@*

@code{caaddr}  @i{x} @result{}  @i{object}
(setf (@code{caaddr} @i{x}) new-object)@*

@code{cadaar}  @i{x} @result{}  @i{object}
(setf (@code{cadaar} @i{x}) new-object)@*

@code{cadadr}  @i{x} @result{}  @i{object}
(setf (@code{cadadr} @i{x}) new-object)@*

@code{caddar}  @i{x} @result{}  @i{object}
(setf (@code{caddar} @i{x}) new-object)@*

@code{cadddr}  @i{x} @result{}  @i{object}
(setf (@code{cadddr} @i{x}) new-object)@*

@code{cdaaar}  @i{x} @result{}  @i{object}
(setf (@code{cdaaar} @i{x}) new-object)@*

@code{cdaadr}  @i{x} @result{}  @i{object}
(setf (@code{cdaadr} @i{x}) new-object)@*

@code{cdadar}  @i{x} @result{}  @i{object}
(setf (@code{cdadar} @i{x}) new-object)@*

@code{cdaddr}  @i{x} @result{}  @i{object}
(setf (@code{cdaddr} @i{x}) new-object)@*

@code{cddaar}  @i{x} @result{}  @i{object}
(setf (@code{cddaar} @i{x}) new-object)@*

@code{cddadr}  @i{x} @result{}  @i{object}
(setf (@code{cddadr} @i{x}) new-object)@*

@code{cdddar}  @i{x} @result{}  @i{object}
(setf (@code{cdddar} @i{x}) new-object)@*

@code{cddddr}  @i{x} @result{}  @i{object}
(setf (@code{cddddr} @i{x}) new-object)@*

@subsubheading  Pronunciation::

@b{cadr}: pronounced 'ka ,de r 

@b{caddr}: pronounced 'kad e ,de r 
  	     or pronounced 'ka ,dude r 

@b{cdr}: pronounced 'ku ,de r 

@b{cddr}: pronounced 'kud e ,de r 
  	    or pronounced 'ke ,dude r 

@subsubheading  Arguments and Values::

@i{x}---a @i{list}.

@i{object}---an @i{object}.

@i{new-object}---an @i{object}.

@subsubheading  Description::

If @i{x} is a @i{cons}, @b{car} returns the @i{car} 
of that @i{cons}.  If @i{x} is @b{nil}, @b{car} returns @b{nil}.

If @i{x} is a @i{cons}, @b{cdr} returns the @i{cdr} 
of that @i{cons}.  If @i{x} is @b{nil}, @b{cdr} returns @b{nil}.

@i{Functions} are provided which perform compositions of up to four
@b{car} and @b{cdr} operations.  Their @i{names} consist of 
a @t{C}, followed by two, three, or four occurrences of @t{A} or @t{D}, 
and finally an @t{R}.  The series of @t{A}'s and @t{D}'s in each
@i{function}'s @i{name} is chosen to identify the series of 
@b{car} and @b{cdr} operations that is performed by the function.
The order in which the @t{A}'s and @t{D}'s appear is the inverse of the
order in which the corresponding operations are performed.  Figure 14--6 
defines the relationships precisely.

@group
@noindent
@w{  This @i{place} ...  Is equivalent to this @i{place} ...  }
@w{  @t{(caar @i{x})}        @t{(car (car @i{x}))}                    }
@w{  @t{(cadr @i{x})}        @t{(car (cdr @i{x}))}                    }
@w{  @t{(cdar @i{x})}        @t{(cdr (car @i{x}))}                    }
@w{  @t{(cddr @i{x})}        @t{(cdr (cdr @i{x}))}                    }
@w{  @t{(caaar @i{x})}       @t{(car (car (car @i{x})))}              }
@w{  @t{(caadr @i{x})}       @t{(car (car (cdr @i{x})))}              }
@w{  @t{(cadar @i{x})}       @t{(car (cdr (car @i{x})))}              }
@w{  @t{(caddr @i{x})}       @t{(car (cdr (cdr @i{x})))}              }
@w{  @t{(cdaar @i{x})}       @t{(cdr (car (car @i{x})))}              }
@w{  @t{(cdadr @i{x})}       @t{(cdr (car (cdr @i{x})))}              }
@w{  @t{(cddar @i{x})}       @t{(cdr (cdr (car @i{x})))}              }
@w{  @t{(cdddr @i{x})}       @t{(cdr (cdr (cdr @i{x})))}              }
@w{  @t{(caaaar @i{x})}      @t{(car (car (car (car @i{x}))))}        }
@w{  @t{(caaadr @i{x})}      @t{(car (car (car (cdr @i{x}))))}        }
@w{  @t{(caadar @i{x})}      @t{(car (car (cdr (car @i{x}))))}        }
@w{  @t{(caaddr @i{x})}      @t{(car (car (cdr (cdr @i{x}))))}        }
@w{  @t{(cadaar @i{x})}      @t{(car (cdr (car (car @i{x}))))}        }
@w{  @t{(cadadr @i{x})}      @t{(car (cdr (car (cdr @i{x}))))}        }
@w{  @t{(caddar @i{x})}      @t{(car (cdr (cdr (car @i{x}))))}        }
@w{  @t{(cadddr @i{x})}      @t{(car (cdr (cdr (cdr @i{x}))))}        }
@w{  @t{(cdaaar @i{x})}      @t{(cdr (car (car (car @i{x}))))}        }
@w{  @t{(cdaadr @i{x})}      @t{(cdr (car (car (cdr @i{x}))))}        }
@w{  @t{(cdadar @i{x})}      @t{(cdr (car (cdr (car @i{x}))))}        }
@w{  @t{(cdaddr @i{x})}      @t{(cdr (car (cdr (cdr @i{x}))))}        }
@w{  @t{(cddaar @i{x})}      @t{(cdr (cdr (car (car @i{x}))))}        }
@w{  @t{(cddadr @i{x})}      @t{(cdr (cdr (car (cdr @i{x}))))}        }
@w{  @t{(cdddar @i{x})}      @t{(cdr (cdr (cdr (car @i{x}))))}        }
@w{  @t{(cddddr @i{x})}      @t{(cdr (cdr (cdr (cdr @i{x}))))}        }

@noindent
@w{         Figure 14--6: CAR and CDR variants        }

@end group

@b{setf} can also be used with any of these functions to change an
existing component of @i{x}, but @b{setf} will not make new
components.  So, for example, the @i{car} of a @i{cons} 
can be assigned with @b{setf} of @b{car},
but the @i{car} of @b{nil} cannot be assigned with @b{setf} of @b{car}.
Similarly, the @i{car} of the @i{car} of a @i{cons} whose @i{car}
is a @i{cons} can be assigned with @b{setf} of @b{caar},
but neither @b{nil} nor a @i{cons} whose car is @b{nil} can be assigned
with @b{setf} of @b{caar}.

The argument @i{x} is permitted to be a @i{dotted list} 
or a @i{circular list}.

@subsubheading  Examples::

@example
 (car nil) @result{}  NIL  
 (cdr '(1 . 2)) @result{}  2
 (cdr '(1 2)) @result{}  (2)
 (cadr '(1 2)) @result{}  2 
 (car '(a b c)) @result{}  A
 (cdr '(a b c)) @result{}  (B C)
@end example

@subsubheading  Exceptional Situations::

The functions @b{car} and @b{cdr} 
should signal @b{type-error} if they receive an argument which is not a
@i{list}.  The other functions (@b{caar}, @b{cadr},
... @b{cddddr}) should behave for the purpose of
error checking as if defined by appropriate calls to @b{car} and
@b{cdr}.

@subsubheading  See Also::

@ref{rplaca; rplacd}
, 
@ref{first; second; third; fourth; fifth; sixth; seventh; eighth; ninth; tenth}
, 
@ref{rest}

@subsubheading  Notes::

The @i{car} of a @i{cons} can also be altered by using @b{rplaca},
and the @i{cdr} of a @i{cons} can be altered by using @b{rplacd}.

@example
(car @i{x})    @equiv{} (first @i{x})
(cadr @i{x})   @equiv{} (second @i{x}) @equiv{} (car (cdr @i{x}))
(caddr @i{x})  @equiv{} (third @i{x})  @equiv{} (car (cdr (cdr @i{x})))
(cadddr @i{x}) @equiv{} (fourth @i{x}) @equiv{} (car (cdr (cdr (cdr @i{x}))))
@end example

@node copy-tree, sublis, car, Conses Dictionary
@subsection copy-tree                                                        [Function]

@code{copy-tree}  @i{tree} @result{}  @i{new-tree}

@subsubheading  Arguments and Values::

@i{tree}---a @i{tree}.

@i{new-tree}---a @i{tree}.

@subsubheading  Description::

Creates a @i{copy} of a @i{tree} of @i{conses}.

If @i{tree} is not a @i{cons}, it is returned; 
otherwise, the result is a new @i{cons} of the results of calling @b{copy-tree} 
on the @i{car} and @i{cdr} of @i{tree}.
In other words, all @i{conses} in the @i{tree} represented by @i{tree}
are copied recursively, stopping only when non-@i{conses} are encountered.

@b{copy-tree} does not preserve circularities and the sharing of substructure.

@subsubheading  Examples::

@example
 (setq object (list (cons 1 "one")
                    (cons 2 (list 'a 'b 'c))))
@result{}  ((1 . "one") (2 A B C))
 (setq object-too object) @result{}  ((1 . "one") (2 A B C))
 (setq copy-as-list (copy-list object))
 (setq copy-as-alist (copy-alist object))
 (setq copy-as-tree (copy-tree object))
 (eq object object-too) @result{}  @i{true}
 (eq copy-as-tree object) @result{}  @i{false}
 (eql copy-as-tree object) @result{}  @i{false}
 (equal copy-as-tree object) @result{}  @i{true}
 (setf (first (cdr (second object))) "a"
       (car (second object)) "two"
       (car object) '(one . 1)) @result{}  (ONE . 1)
 object @result{}  ((ONE . 1) ("two" "a" B C))
 object-too @result{}  ((ONE . 1) ("two" "a" B C))
 copy-as-list @result{}  ((1 . "one") ("two" "a" B C))
 copy-as-alist @result{}  ((1 . "one") (2 "a" B C))
 copy-as-tree @result{}  ((1 . "one") (2 A B C)) 
@end example

@subsubheading  See Also::

@ref{tree-equal}

@node sublis, subst, copy-tree, Conses Dictionary
@subsection sublis, nsublis                                                  [Function]

@code{sublis}  @i{alist tree {&key} key test test-not} @result{}  @i{new-tree}

@code{nsublis}  @i{alist tree {&key} key test test-not} @result{}  @i{new-tree}

@subsubheading  Arguments and Values::

@i{alist}---an @i{association list}.

@i{tree}---a @i{tree}.

@i{test}---a @i{designator} for a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{test-not}---a @i{designator} for 
  a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{key}---a @i{designator} for a @i{function} of one argument,
  or @b{nil}.

@i{new-tree}---a @i{tree}.

@subsubheading  Description::

@b{sublis} makes substitutions for @i{objects} in @i{tree}
(a structure of @i{conses}).
@b{nsublis} is like @b{sublis} 
but destructively modifies the relevant
parts of the @i{tree}.

@b{sublis} looks at all subtrees and leaves of @i{tree};
if a subtree or leaf appears as a key in @i{alist}
(that is, the key and the subtree or leaf @i{satisfy the test}),
it is replaced by the @i{object} with which that key is associated.
This operation is non-destructive.  In effect, @b{sublis} can
perform several @b{subst} operations simultaneously.

If @b{sublis} succeeds, a new copy of @i{tree} is returned in
which each occurrence of such a subtree or leaf is replaced by the
@i{object} with which it is associated.   If no changes are made, the
original tree is returned.  The original @i{tree} is left unchanged,
but the result tree may share cells with it.

@b{nsublis} is permitted to modify @i{tree} 
but otherwise returns the same values as @b{sublis}.

@subsubheading  Examples::

@example
 (sublis '((x . 100) (z . zprime))
         '(plus x (minus g z x p) 4 . x))
@result{}  (PLUS 100 (MINUS G ZPRIME 100 P) 4 . 100)
 (sublis '(((+ x y) . (- x y)) ((- x y) . (+ x y)))
         '(* (/ (+ x y) (+ x p)) (- x y))
         :test #'equal)
@result{}  (* (/ (- X Y) (+ X P)) (+ X Y))
 (setq tree1 '(1 (1 2) ((1 2 3)) (((1 2 3 4)))))
@result{}  (1 (1 2) ((1 2 3)) (((1 2 3 4))))
 (sublis '((3 . "three")) tree1) 
@result{}  (1 (1 2) ((1 2 "three")) (((1 2 "three" 4))))
 (sublis '((t . "string"))
          (sublis '((1 . "") (4 . 44)) tree1)
          :key #'stringp)
@result{}  ("string" ("string" 2) (("string" 2 3)) ((("string" 2 3 44))))
 tree1 @result{}  (1 (1 2) ((1 2 3)) (((1 2 3 4))))
 (setq tree2 '("one" ("one" "two") (("one" "Two" "three"))))
@result{}  ("one" ("one" "two") (("one" "Two" "three"))) 
 (sublis '(("two" . 2)) tree2) 
@result{}  ("one" ("one" "two") (("one" "Two" "three"))) 
 tree2 @result{}  ("one" ("one" "two") (("one" "Two" "three"))) 
 (sublis '(("two" . 2)) tree2 :test 'equal) 
@result{}  ("one" ("one" 2) (("one" "Two" "three"))) 

 (nsublis '((t . 'temp))
           tree1
           :key #'(lambda (x) (or (atom x) (< (list-length x) 3))))
@result{}  ((QUOTE TEMP) (QUOTE TEMP) QUOTE TEMP) 
@end example

@subsubheading  Side Effects::

@b{nsublis} modifies @i{tree}.

@subsubheading  See Also::

@ref{subst; subst-if; subst-if-not; nsubst; nsubst-if; nsubst-if-not}
,

@ref{Compiler Terminology},

@ref{Traversal Rules and Side Effects}

@subsubheading  Notes::

The @t{:test-not} parameter is deprecated.

Because the side-effecting variants (@i{e.g.}, @b{nsublis}) potentially
change the path that is being traversed, their effects in the presence
of shared or circular structure structure may vary in surprising ways
when compared to their non-side-effecting alternatives.  To see this,
consider the following side-effect behavior, which might be exhibited by
some implementations:

@example
 (defun test-it (fn)
   (let* ((shared-piece (list 'a 'b))
          (data (list shared-piece shared-piece)))
     (funcall fn '((a . b) (b . a)) data)))
 (test-it #'sublis) @result{}  ((B A) (B A))
 (test-it #'nsublis) @result{}  ((A B) (A B))
@end example

@node subst, tree-equal, sublis, Conses Dictionary
@subsection subst, subst-if, subst-if-not, nsubst, nsubst-if, nsubst-if-not
@flushright
@i{[Function]}
@end flushright

@code{subst}  @i{new old  tree {&key} key test test-not} @result{}  @i{new-tree}

@code{subst-if}  @i{new predicate tree {&key} key} @result{}  @i{new-tree}

@code{subst-if-not}  @i{new predicate tree {&key} key} @result{}  @i{new-tree}

@code{nsubst}  @i{new old  tree {&key} key test test-not} @result{}  @i{new-tree}

@code{nsubst-if}  @i{new predicate tree {&key} key} @result{}  @i{new-tree}

@code{nsubst-if-not}  @i{new predicate tree {&key} key} @result{}  @i{new-tree}

@subsubheading  Arguments and Values:: 

@i{new}---an @i{object}.

@i{old}---an @i{object}.

@i{predicate}---a @i{symbol} that names a @i{function},
   or a @i{function} of one argument 
      that returns a @i{generalized boolean} value.

@i{tree}---a @i{tree}.

@i{test}---a @i{designator} for a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{test-not}---a @i{designator} for 
  a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{key}---a @i{designator} for a @i{function} of one argument,
  or @b{nil}.

@i{new-tree}---a @i{tree}.

@subsubheading  Description::

@b{subst}, @b{subst-if}, and @b{subst-if-not} perform
substitution operations on @i{tree}.  
Each function searches @i{tree} for occurrences of a 
particular @i{old} item of an element or subexpression that 
@i{satisfies the test}.

@b{nsubst}, @b{nsubst-if}, and @b{nsubst-if-not} are 
like @b{subst},
@b{subst-if}, and @b{subst-if-not} respectively, except that the 
original @i{tree} is  modified.

@b{subst} makes a copy of @i{tree},
substituting @i{new} for every subtree or leaf of @i{tree}
(whether the subtree or leaf is a @i{car} or a @i{cdr} of its parent)
such that @i{old} and the subtree or leaf @i{satisfy the test}.  

@b{nsubst} is a destructive version of @b{subst}.  
The list structure of
@i{tree} is altered by destructively replacing with @i{new}
each leaf of the @i{tree} such that @i{old} and the leaf
@i{satisfy the test}.

For @b{subst}, @b{subst-if}, 
and @b{subst-if-not},
if the functions succeed, a new
copy of the tree is returned in which each  occurrence of such an
element is replaced by the
@i{new} element or subexpression.  If no changes are made, the original 
@i{tree} may be returned.
The original @i{tree} is left unchanged, but the result tree 
may share storage with it.           

For @b{nsubst}, @b{nsubst-if}, 
and @b{nsubst-if-not}
the original @i{tree} is  modified and returned as the function result,
but the result may not be @b{eq} to @i{tree}.

@subsubheading  Examples::

@example
 (setq tree1 '(1 (1 2) (1 2 3) (1 2 3 4))) @result{}  (1 (1 2) (1 2 3) (1 2 3 4))
 (subst "two" 2 tree1) @result{}  (1 (1 "two") (1 "two" 3) (1 "two" 3 4))
 (subst "five" 5 tree1) @result{}  (1 (1 2) (1 2 3) (1 2 3 4))
 (eq tree1 (subst "five" 5 tree1)) @result{}  @i{implementation-dependent}
 (subst 'tempest 'hurricane
        '(shakespeare wrote (the hurricane)))
@result{}  (SHAKESPEARE WROTE (THE TEMPEST))
 (subst 'foo 'nil '(shakespeare wrote (twelfth night)))
@result{}  (SHAKESPEARE WROTE (TWELFTH NIGHT . FOO) . FOO)
 (subst '(a . cons) '(old . pair)
        '((old . spice) ((old . shoes) old . pair) (old . pair))
        :test #'equal)
@result{}  ((OLD . SPICE) ((OLD . SHOES) A . CONS) (A . CONS))

 (subst-if 5 #'listp tree1) @result{}  5
 (subst-if-not '(x) #'consp tree1) 
@result{}  (1 X)

 tree1 @result{}  (1 (1 2) (1 2 3) (1 2 3 4))
 (nsubst 'x 3 tree1 :key #'(lambda (y) (and (listp y) (third y)))) 
@result{}  (1 (1 2) X X)
 tree1 @result{}  (1 (1 2) X X)
@end example

@subsubheading  Side Effects::

@b{nsubst}, @b{nsubst-if}, and @b{nsubst-if-not} 
might alter the @i{tree structure} of @i{tree}.

@subsubheading  See Also::

@ref{substitute; substitute-if; substitute-if-not; nsubstitute; nsubstitute-if; nsubstitute-if-not}
,
@b{nsubstitute},

@ref{Compiler Terminology},

@ref{Traversal Rules and Side Effects}

@subsubheading  Notes::

The @t{:test-not} parameter is deprecated.

The functions @b{subst-if-not} and @b{nsubst-if-not} are deprecated.

One possible definition of @b{subst}:

@example
 (defun subst (old new tree &rest x &key test test-not key)
   (cond ((satisfies-the-test old tree :test test
                                 :test-not test-not :key key)
         new)
        ((atom tree) tree)
        (t (let ((a (apply #'subst old new (car tree) x))
                 (d (apply #'subst old new (cdr tree) x)))
             (if (and (eql a (car tree))
                      (eql d (cdr tree)))
                 tree
                 (cons a d))))))
@end example

@node tree-equal, copy-list, subst, Conses Dictionary
@subsection tree-equal                                                       [Function]

@code{tree-equal}  @i{tree-1 tree-2 {&key} test test-not} @result{}  @i{generalized-boolean}

@subsubheading  Arguments and Values:: 

@i{tree-1}---a @i{tree}.

@i{tree-2}---a @i{tree}.

@i{test}---a @i{designator} for a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{test-not}---a @i{designator} for 
  a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{generalized-boolean}---a @i{generalized boolean}.

@subsubheading  Description::

@b{tree-equal} tests whether two trees are of the same shape
and have the same leaves.  
@b{tree-equal} returns @i{true} if @i{tree-1} and @i{tree-2}  are 
both @i{atoms} and @i{satisfy the test},
or if they are both @i{conses} and
the @i{car} of @i{tree-1} is @b{tree-equal} to
the @i{car} of @i{tree-2} and
the @i{cdr} of @i{tree-1} is @b{tree-equal} to
the @i{cdr} of @i{tree-2}.  
Otherwise, @b{tree-equal} returns @i{false}.

@b{tree-equal} recursively compares @i{conses} but not any 
other @i{objects} that have components.  

The first argument to the @t{:test} or @t{:test-not} 
function is @i{tree-1} or a @i{car} or @i{cdr} of @i{tree-1};
the second argument is @i{tree-2} or a @i{car} 
or @i{cdr} of @i{tree-2}.

@subsubheading  Examples::

@example
 (setq tree1 '(1 (1 2))
       tree2 '(1 (1 2))) @result{}  (1 (1 2))
 (tree-equal tree1 tree2) @result{}  @i{true}
 (eql tree1 tree2) @result{}  @i{false}
 (setq tree1 '('a ('b 'c))
       tree2 '('a ('b 'c))) @result{}  ('a ('b 'c)) 
@result{}  ((QUOTE A) ((QUOTE B) (QUOTE C)))
 (tree-equal tree1 tree2 :test 'eq) @result{}  @i{true}
@end example

@subsubheading  Exceptional Situations::

The consequences are undefined 
if both @i{tree-1} and @i{tree-2} are circular.

@subsubheading  See Also::

@ref{equal}
,

@ref{Traversal Rules and Side Effects}

@subsubheading  Notes::

The @t{:test-not} parameter is deprecated.

@node copy-list, list, tree-equal, Conses Dictionary
@subsection copy-list                                                        [Function]

@code{copy-list}  @i{list} @result{}  @i{copy}

@subsubheading  Arguments and Values::

@i{list}---a @i{proper list} or a @i{dotted list}.

@i{copy}---a @i{list}.

@subsubheading  Description::

Returns a @i{copy} of @i{list}.
If @i{list} is a @i{dotted list},
the resulting @i{list} will also be a @i{dotted list}.

Only the @i{list structure} of @i{list} is copied; 
the @i{elements} of the resulting list are 
the @i{same} as the corresponding @i{elements} of the given @i{list}.

@subsubheading  Examples::    

@example
 (setq lst (list 1 (list 2 3))) @result{}  (1 (2 3))
 (setq slst lst) @result{}  (1 (2 3))
 (setq clst (copy-list lst)) @result{}  (1 (2 3))
 (eq slst lst) @result{}  @i{true}
 (eq clst lst) @result{}  @i{false}
 (equal clst lst) @result{}  @i{true}
 (rplaca lst "one") @result{}  ("one" (2 3))
 slst @result{}  ("one" (2 3))
 clst @result{}  (1 (2 3))
 (setf (caadr lst) "two") @result{}  "two"
 lst @result{}  ("one" ("two" 3))
 slst @result{}  ("one" ("two" 3))
 clst @result{}  (1 ("two" 3))
@end example

@subsubheading  Exceptional Situations::

The consequences are undefined if @i{list} is a @i{circular list}.

@subsubheading  See Also::

@ref{copy-alist}
,
@ref{copy-seq}
,
@ref{copy-tree}

@subsubheading  Notes::

The copy created is @b{equal} to @i{list}, but not @b{eq}.

@node list, list-length, copy-list, Conses Dictionary
@subsection list, list*                                                      [Function]

@code{list}  @i{{&rest} objects} @result{}  @i{list}

@code{list*}  @i{{&rest} objects^+} @result{}  @i{result}

@subsubheading  Arguments and Values:: 

@i{object}---an @i{object}.

@i{list}---a @i{list}.

@i{result}---an @i{object}.

@subsubheading  Description::

@b{list} returns a @i{list} containing the supplied @i{objects}.  

@b{list*} is like @b{list} except that 
the last @i{argument} to @b{list} becomes 
the @i{car} of the last @i{cons} constructed, while
the last @i{argument} to @b{list*} becomes 
the @i{cdr} of the last @i{cons} constructed.
Hence, any given call to @b{list*} always produces one fewer @i{conses}
than a call to @b{list} with the same number of arguments.

If the last @i{argument} to @b{list*} is a @i{list}, 
the effect is to construct a new @i{list} which is similar, but
which has additional elements added to the front corresponding to
the preceding @i{arguments} of @b{list*}.

If @b{list*} receives only one @i{object},
that @i{object} is returned, regardless of whether or not it is a @i{list}.

@subsubheading  Examples::

@example
 (list 1) @result{}  (1)
 (list* 1) @result{}  1
 (setq a 1) @result{}  1
 (list a 2) @result{}  (1 2)
 '(a 2) @result{}  (A 2)
 (list 'a 2) @result{}  (A 2)
 (list* a 2) @result{}  (1 . 2)
 (list) @result{}  NIL ;@i{i.e.}, ()
 (setq a '(1 2)) @result{}  (1 2)
 (eq a (list* a)) @result{}  @i{true}
 (list 3 4 'a (car '(b . c)) (+ 6 -2)) @result{}  (3 4 A B 4)
 (list* 'a 'b 'c 'd) @equiv{} (cons 'a (cons 'b (cons 'c 'd))) @result{}  (A B C . D)
 (list* 'a 'b 'c '(d e f)) @result{}  (A B C D E F)
@end example

@subsubheading  See Also::

@ref{cons}

@subsubheading  Notes::

@example
 (list* @i{x}) @equiv{} @i{x}
@end example

@node list-length, listp, list, Conses Dictionary
@subsection list-length                                                      [Function]

@code{list-length}  @i{list} @result{}  @i{length}

@subsubheading  Arguments and Values::

@i{list}---a @i{proper list} or a @i{circular list}.

@i{length}---a non-negative @i{integer}, or @b{nil}.

@subsubheading  Description::

Returns the @i{length} of @i{list} if @i{list} is a @i{proper list}.
Returns @b{nil} if @i{list} is a @i{circular list}.

@subsubheading  Examples::

@example
 (list-length '(a b c d)) @result{}  4
 (list-length '(a (b c) d)) @result{}  3
 (list-length '()) @result{}  0
 (list-length nil) @result{}  0
 (defun circular-list (&rest elements)
   (let ((cycle (copy-list elements))) 
     (nconc cycle cycle)))
 (list-length (circular-list 'a 'b)) @result{}  NIL
 (list-length (circular-list 'a)) @result{}  NIL
 (list-length (circular-list)) @result{}  0
@end example

@subsubheading  Exceptional Situations::

Should signal an error of @i{type} @b{type-error}
			      if @i{list} is not a @i{proper list} or a @i{circular list}.

@subsubheading  See Also::

@ref{length}

@subsubheading  Notes::

@b{list-length} could be implemented as follows:

@example
 (defun list-length (x)  
   (do ((n 0 (+ n 2))           ;Counter.
        (fast x (cddr fast))    ;Fast pointer: leaps by 2.
        (slow x (cdr slow)))    ;Slow pointer: leaps by 1.
       (nil)
     ;; If fast pointer hits the end, return the count.
     (when (endp fast) (return n))
     (when (endp (cdr fast)) (return (+ n 1)))
     ;; If fast pointer eventually equals slow pointer,
     ;;  then we must be stuck in a circular list.
     ;; (A deeper property is the converse: if we are
     ;;  stuck in a circular list, then eventually the
     ;;  fast pointer will equal the slow pointer.
     ;;  That fact justifies this implementation.)
     (when (and (eq fast slow) (> n 0)) (return nil))))

@end example

@node listp, make-list, list-length, Conses Dictionary
@subsection listp                                                            [Function]

@code{listp}  @i{object} @result{}  @i{generalized-boolean}

@subsubheading  Arguments and Values::

@i{object}---an @i{object}.

@i{generalized-boolean}---a @i{generalized boolean}.

@subsubheading  Description::

Returns @i{true} if @i{object} is of @i{type} @b{list};
otherwise, returns @i{false}.

@subsubheading  Examples::
@example
 (listp nil) @result{}  @i{true}
 (listp (cons 1 2)) @result{}  @i{true}
 (listp (make-array 6)) @result{}  @i{false}
 (listp t) @result{}  @i{false}
@end example

@subsubheading  See Also::

@ref{consp}

@subsubheading  Notes::

If @i{object} is a @i{cons},
@b{listp} does not check whether @i{object} is a @i{proper list};
it returns @i{true} for any kind of @i{list}.

@example
 (listp @i{object}) @equiv{} (typep @i{object} 'list) @equiv{} (typep @i{object} '(or cons null))
@end example

@node make-list, push, listp, Conses Dictionary
@subsection make-list                                                        [Function]

@code{make-list}  @i{size {&key} initial-element} @result{}  @i{list}

@subsubheading  Arguments and Values::

@i{size}---a non-negative @i{integer}.

@i{initial-element}---an @i{object}.
  The default is @b{nil}.

@i{list}---a @i{list}.

@subsubheading  Description::

Returns a @i{list} of @i{length} given by @i{size},
each of the @i{elements} of which is @i{initial-element}.

@subsubheading  Examples::
@example
 (make-list 5) @result{}  (NIL NIL NIL NIL NIL)
 (make-list 3 :initial-element 'rah) @result{}  (RAH RAH RAH)
 (make-list 2 :initial-element '(1 2 3)) @result{}  ((1 2 3) (1 2 3))
 (make-list 0) @result{}  NIL ;@i{i.e.}, ()
 (make-list 0 :initial-element 'new-element) @result{}  NIL 
@end example

@subsubheading  Exceptional Situations::

Should signal an error of @i{type} @b{type-error}
			      if @i{size} is not a non-negative @i{integer}.

@subsubheading  See Also::

@ref{cons}
,
@ref{list}

@node push, pop, make-list, Conses Dictionary
@subsection push                                                                [Macro]

@code{push}  @i{item place} @result{}  @i{new-place-value}

@subsubheading  Arguments and Values::

@i{item}---an @i{object}. 

@i{place}---a @i{place}, the @i{value} of which may be any @i{object}.  

@i{new-place-value}---a @i{list} (the new @i{value} of @i{place}).

@subsubheading  Description::

@b{push} prepends @i{item} to the @i{list} that is stored
in @i{place}, stores the resulting @i{list} in @i{place}, 
and returns the @i{list}.

For information about the @i{evaluation} of @i{subforms} of @i{place},
see @ref{Evaluation of Subforms to Places}.

@subsubheading  Examples::
@example
 (setq llst '(nil)) @result{}  (NIL)
 (push 1 (car llst)) @result{}  (1)
 llst @result{}  ((1))
 (push 1 (car llst)) @result{}  (1 1)
 llst @result{}  ((1 1))
 (setq x '(a (b c) d)) @result{}  (A (B C) D)
 (push 5 (cadr x)) @result{}  (5 B C)  
 x @result{}  (A (5 B C) D)
@end example

@subsubheading  Side Effects::

The contents of @i{place} are modified.

@subsubheading  See Also::

@ref{pop}
,
@ref{pushnew}
,
@ref{Generalized Reference}

@subsubheading  Notes::
The effect of @t{(push @i{item} @i{place})}
is equivalent to

@example
 (setf place (cons @i{item} @i{place}))
@end example

except that the @i{subforms} of @i{place} 
are evaluated only once, and @i{item} is evaluated
before @i{place}.

@node pop, first, push, Conses Dictionary
@subsection pop                                                                 [Macro]

@code{pop}  @i{place} @result{}  @i{element}

@subsubheading  Arguments and Values:: 

@i{place}---a @i{place}, the @i{value} of which is a @i{list}
   (possibly, but necessarily, a @i{dotted list} or @i{circular list}).

@i{element}---an @i{object} (the @i{car} of the contents of @i{place}).

@subsubheading  Description::

@b{pop} @i{reads} the @i{value} of @i{place},
remembers the @i{car} of the @i{list} which was retrieved,
@i{writes} the @i{cdr} of the @i{list} back into the @i{place},
and finally @i{yields} the @i{car} of the originally retrieved @i{list}.

For information about the @i{evaluation} of @i{subforms} of @i{place},
see @ref{Evaluation of Subforms to Places}.

@subsubheading  Examples::

@example
 (setq stack '(a b c)) @result{}  (A B C)
 (pop stack) @result{}  A  
 stack @result{}  (B C)
 (setq llst '((1 2 3 4))) @result{}  ((1 2 3 4))
 (pop (car llst)) @result{}  1
 llst @result{}  ((2 3 4))
@end example

@subsubheading  Side Effects::

The contents of @i{place} are modified.

@subsubheading  See Also::

@ref{push}
,
@ref{pushnew}
,
@ref{Generalized Reference}

@subsubheading  Notes::

The effect of @t{(pop @i{place})} is roughly equivalent to

@example
 (prog1 (car @i{place}) (setf @i{place} (cdr @i{place})))
@end example

except that the latter would evaluate any @i{subforms} of @i{place}
three times, while @b{pop} evaluates them only once.

@node first, nth, pop, Conses Dictionary
@subsection first, second, third, fourth, fifth, 
@subheading sixth, seventh, eighth, ninth, tenth
@flushright
@i{[Accessor]}
@end flushright

@code{first}  @i{list} @result{}  @i{object}
(setf (@code{first} @i{list}) new-object)@*

@code{second}  @i{list} @result{}  @i{object}
(setf (@code{second} @i{list}) new-object)@*

@code{third}  @i{list} @result{}  @i{object}
(setf (@code{third} @i{list}) new-object)@*

@code{fourth}  @i{list} @result{}  @i{object}
(setf (@code{fourth} @i{list}) new-object)@*

@code{fifth}  @i{list} @result{}  @i{object}
(setf (@code{fifth} @i{list}) new-object)@*

@code{sixth}  @i{list} @result{}  @i{object}
(setf (@code{sixth} @i{list}) new-object)@*

@code{seventh}  @i{list} @result{}  @i{object}
(setf (@code{seventh} @i{list}) new-object)@*

@code{eighth}  @i{list} @result{}  @i{object}
(setf (@code{eighth} @i{list}) new-object)@*

@code{ninth}  @i{list} @result{}  @i{object}
(setf (@code{ninth} @i{list}) new-object)@*

@code{tenth}  @i{list} @result{}  @i{object}
(setf (@code{tenth} @i{list}) new-object)@*

@subsubheading  Arguments and Values::

@i{list}---a @i{list}, 

	       which might be a @i{dotted list} or a @i{circular list}.

@i{object}, @i{new-object}---an @i{object}.

@subsubheading  Description::

The functions
@b{first},
@b{second},
@b{third}, 
@b{fourth}, 
@b{fifth},
@b{sixth},
@b{seventh},
@b{eighth}, 
@b{ninth},
and
@b{tenth}
@i{access} the first, second, third, fourth, fifth, sixth, seventh, eighth,
ninth, and tenth @i{elements} of @i{list}, respectively.
Specifically,

@example
 (first @i{list})    @equiv{}  (car @i{list})
 (second @i{list})   @equiv{}  (car (cdr @i{list}))
 (third @i{list})    @equiv{}  (car (cddr @i{list}))
 (fourth @i{list})   @equiv{}  (car (cdddr @i{list}))
 (fifth @i{list})    @equiv{}  (car (cddddr @i{list}))
 (sixth @i{list})    @equiv{}  (car (cdr (cddddr @i{list})))
 (seventh @i{list})  @equiv{}  (car (cddr (cddddr @i{list})))
 (eighth @i{list})   @equiv{}  (car (cdddr (cddddr @i{list})))
 (ninth @i{list})    @equiv{}  (car (cddddr (cddddr @i{list})))
 (tenth @i{list})    @equiv{}  (car (cdr (cddddr (cddddr @i{list}))))
@end example

@b{setf} can also be used with any of these functions to change an
existing component.  The same equivalences apply.  For example:

@example
 (setf (fifth @i{list}) @i{new-object}) @equiv{} (setf (car (cddddr @i{list})) @i{new-object})
@end example

@subsubheading  Examples::

@example
 (setq lst '(1 2 3 (4 5 6) ((V)) vi 7 8 9 10)) 
@result{}  (1 2 3 (4 5 6) ((V)) VI 7 8 9 10)
 (first lst) @result{}  1
 (tenth lst) @result{}  10
 (fifth lst) @result{}  ((V))
 (second (fourth lst)) @result{}  5
 (sixth '(1 2 3)) @result{}  NIL
 (setf (fourth lst) "four") @result{}  "four"
 lst @result{}  (1 2 3 "four" ((V)) VI 7 8 9 10)
@end example

@subsubheading  See Also::

@ref{car; cdr; caar; cadr; cdar; cddr; caaar; caadr; cadar; caddr; cdaar; cdadr; cddar; cdddr; caaaar; caaadr; caadar; caaddr; cadaar; cadadr; caddar; cadddr; cdaaar; cdaadr; cdadar; cdaddr; cddaar; cddadr; cdddar; cddddr}
, 
@ref{nth}

@subsubheading  Notes::

@b{first}  is functionally equivalent to @b{car},
@b{second} is functionally equivalent to @b{cadr},
@b{third}  is functionally equivalent to @b{caddr}, and 
@b{fourth} is functionally equivalent to @b{cadddr}.

The ordinal numbering used here is one-origin,
as opposed to the zero-origin numbering used by @b{nth}:

@example
 (fifth x) @equiv{} (nth 4 x)
@end example

@node nth, endp, first, Conses Dictionary
@subsection nth                                                              [Accessor]

@code{nth}  @i{n list} @result{}  @i{object}

(setf (@code{         nth} @i{n list}) new-object)@*

@subsubheading  Arguments and Values::

@i{n}---a non-negative @i{integer}.

@i{list}---a @i{list},

	       which might be a @i{dotted list} or a @i{circular list}.

@i{object}---an @i{object}.

@i{new-object}---an @i{object}.

@subsubheading  Description::

@b{nth} locates the @i{n}th element of @i{list},
where the @i{car} of the @i{list} is the ``zeroth'' element.

Specifically,

@example
 (nth @i{n} @i{list}) @equiv{} (car (nthcdr @i{n} @i{list}))
@end example

@b{nth} may be used to specify a @i{place} to @b{setf}.

Specifically,

@example
 (setf (nth @i{n} @i{list}) @i{new-object}) @equiv{} (setf (car (nthcdr @i{n} @i{list})) @i{new-object})
@end example

@subsubheading  Examples::

@example
 (nth 0 '(foo bar baz)) @result{}  FOO
 (nth 1 '(foo bar baz)) @result{}  BAR
 (nth 3 '(foo bar baz)) @result{}  NIL
 (setq 0-to-3 (list 0 1 2 3)) @result{}  (0 1 2 3)
 (setf (nth 2 0-to-3) "two") @result{}  "two"
 0-to-3 @result{}  (0 1 "two" 3)
@end example

@subsubheading  See Also::

@ref{elt}
,
@ref{first; second; third; fourth; fifth; sixth; seventh; eighth; ninth; tenth}
,
@ref{nthcdr}

@node endp, null, nth, Conses Dictionary
@subsection endp                                                             [Function]

@code{endp}  @i{list} @result{}  @i{generalized-boolean}

@subsubheading  Arguments and Values::

@i{list}---a @i{list},

	       which might be a @i{dotted list} or a @i{circular list}.

@i{generalized-boolean}---a @i{generalized boolean}.

@subsubheading  Description::

Returns @i{true}  if @i{list} is the @i{empty list}.
Returns @i{false} if @i{list} is a @i{cons}.

@subsubheading  Examples::

@example
 (endp nil) @result{}  @i{true}
 (endp '(1 2)) @result{}  @i{false}
 (endp (cddr '(1 2))) @result{}  @i{true}
@end example

@subsubheading  Exceptional Situations::

Should signal an error of @i{type} @b{type-error}
			      if @i{list} is not a @i{list}.

@subsubheading  Notes::

The purpose of @b{endp} is to test for the end of @i{proper list}.
Since @b{endp} does not descend into a @i{cons}, 
it is well-defined to pass it a @i{dotted list}.
However, if shorter ``lists'' are iteratively produced 
by calling @b{cdr} on such a @i{dotted list} 
and those ``lists'' are tested with @b{endp},
a situation that has undefined consequences will eventually result 
when the @i{non-nil} @i{atom} (which is not in fact a @i{list})
finally becomes the argument to @b{endp}.
Since this is the usual way in which @b{endp} is used,
it is conservative programming style 
and consistent with the intent of @b{endp}
to treat @b{endp} as simply a function on @i{proper lists} 
which happens not to enforce an argument type of @i{proper list} except
when the argument is @i{atomic}.

@node null, nconc, endp, Conses Dictionary
@subsection null                                                             [Function]

@code{null}  @i{object} @result{}  @i{boolean}

@subsubheading  Arguments and Values::

@i{object}---an @i{object}.

@i{boolean}---a @i{boolean}.

@subsubheading  Description::

Returns @b{t} if @i{object} is the @i{empty list};
otherwise, returns @b{nil}.

@subsubheading  Examples::   

@example
 (null '()) @result{}  T
 (null nil) @result{}  T
 (null t) @result{}  NIL
 (null 1) @result{}  NIL
@end example

@subsubheading  See Also::

@ref{not}

@subsubheading  Notes::

@b{null} is intended to be used to test for the @i{empty list}
whereas @b{not} is intended to be used to invert a @i{boolean}
(or @i{generalized boolean}).
Operationally, @b{null} and @b{not} compute the same result;
which to use is a matter of style.

@example
 (null @i{object}) @equiv{} (typep @i{object} 'null) @equiv{} (eq @i{object} '@t{()})
@end example

@node nconc, append, null, Conses Dictionary
@subsection nconc                                                            [Function]

@code{nconc}  @i{{&rest} lists} @result{}  @i{concatenated-list}

@subsubheading  Arguments and Values::

@i{list}---each but the last must be a @i{list} 
	(which might be a @i{dotted list} but must not be a @i{circular list});
        the last @i{list} may be any @i{object}.

@i{concatenated-list}---a @i{list}.

@subsubheading  Description::

Returns a @i{list} that is the concatenation of @i{lists}.
If no @i{lists} are supplied, @t{(nconc)} returns @b{nil}.

@b{nconc} is defined using the following recursive relationship:

@example
 (nconc) @result{}  ()
 (nconc nil . @i{lists}) @equiv{} (nconc . @i{lists})
 (nconc @i{list}) @result{}  @i{list}
 (nconc @i{list-1} @i{list-2}) @equiv{} (progn (rplacd (last @i{list-1}) @i{list-2}) @i{list-1})
 (nconc @i{list-1} @i{list-2} . @i{lists}) @equiv{} (nconc (nconc @i{list-1} @i{list-2}) . @i{lists})
@end example

@subsubheading  Examples::

@example
 (nconc) @result{}  NIL
 (setq x '(a b c)) @result{}  (A B C)
 (setq y '(d e f)) @result{}  (D E F)
 (nconc x y) @result{}  (A B C D E F)
 x @result{}  (A B C D E F)
@end example

Note, in the example, that the value of @t{x} is now different,
since its last @i{cons} 
has been @b{rplacd}'d to the value of @t{y}.
If @t{(nconc x y)} were evaluated again,
it would yield a piece of a @i{circular list},
whose printed representation would be
@t{(A B C D E F D E F D E F ...)}, repeating forever;
if the @b{*print-circle*} switch were @i{non-nil},
it would be printed as @t{(A B C . #1=(D E F . #1#))}.

@example
 (setq foo (list 'a 'b 'c 'd 'e)
       bar (list 'f 'g 'h 'i 'j)
       baz (list 'k 'l 'm)) @result{}  (K L M)
 (setq foo (nconc foo bar baz)) @result{}  (A B C D E F G H I J K L M)
 foo @result{}  (A B C D E F G H I J K L M)
 bar @result{}  (F G H I J K L M)
 baz @result{}  (K L M)

 (setq foo (list 'a 'b 'c 'd 'e)
       bar (list 'f 'g 'h 'i 'j)
       baz (list 'k 'l 'm)) @result{}  (K L M)
 (setq foo (nconc nil foo bar nil baz)) @result{}  (A B C D E F G H I J K L M) 
 foo @result{}  (A B C D E F G H I J K L M)
 bar @result{}  (F G H I J K L M)
 baz @result{}  (K L M)
@end example

@subsubheading  Side Effects::

The @i{lists} are modified rather than copied.

@subsubheading  See Also::

@ref{append}
, 
@ref{concatenate}

@node append, revappend, nconc, Conses Dictionary
@subsection append                                                           [Function]

@code{append}  @i{{&rest} lists} @result{}  @i{result}

@subsubheading  Arguments and Values:: 

@i{list}---each must be a @i{proper list} except the last,
	       which may be any @i{object}.

@i{result}---an @i{object}.  This will be a @i{list}
		 unless the last @i{list} was not a @i{list}
        	    and all preceding @i{lists} were @i{null}.

@subsubheading  Description::

@b{append} returns a new @i{list} that is the concatenation of
the copies.  @i{lists} are left unchanged; the @i{list structure} 
of each of @i{lists} except the last is copied.
The last argument is not copied; it becomes the @i{cdr} of the 
final @i{dotted pair} of the concatenation of the preceding @i{lists},
or is returned directly if there are no preceding
@i{non-empty} 
@i{lists}.

@subsubheading  Examples::

@example
 (append '(a b c) '(d e f) '() '(g)) @result{}  (A B C D E F G)
 (append '(a b c) 'd) @result{}  (A B C . D)
 (setq lst '(a b c)) @result{}  (A B C)
 (append lst '(d)) @result{}  (A B C D)
 lst @result{}  (A B C)
 (append) @result{}  NIL
 (append 'a) @result{}  A
@end example

@subsubheading  See Also::

@ref{nconc}
, 
@ref{concatenate}

@node revappend, butlast, append, Conses Dictionary
@subsection revappend, nreconc                                               [Function]

@code{revappend}  @i{list tail} @result{}  @i{result-list}

@code{nreconc}  @i{list tail} @result{}  @i{result-list}

@subsubheading  Arguments and Values::

@i{list}---a @i{proper list}.

@i{tail}---an @i{object}. 

@i{result-list}---an @i{object}.

@subsubheading  Description::

@b{revappend} constructs a @i{copy}_2 of @i{list},
but with the @i{elements} in reverse order.  It then appends (as if
by @b{nconc}) the @i{tail} to that reversed list and returns the result.

@b{nreconc} reverses the order of @i{elements} in @i{list}
(as if by @b{nreverse}).  It then appends (as if by @b{nconc})
the @i{tail} to that reversed list and returns the result.

The resulting @i{list} shares @i{list structure} with @i{tail}.

@subsubheading  Examples::

@example
 (let ((list-1 (list 1 2 3))
       (list-2 (list 'a 'b 'c)))
   (print (revappend list-1 list-2))
   (print (equal list-1 '(1 2 3)))
   (print (equal list-2 '(a b c))))
@t{ |> } (3 2 1 A B C) 
@t{ |> } T
@t{ |> } T
@result{}  T

 (revappend '(1 2 3) '()) @result{}  (3 2 1)
 (revappend '(1 2 3) '(a . b)) @result{}  (3 2 1 A . B)
 (revappend '() '(a b c)) @result{}  (A B C)
 (revappend '(1 2 3) 'a) @result{}  (3 2 1 . A)
 (revappend '() 'a) @result{}  A   ;degenerate case

 (let ((list-1 '(1 2 3))
       (list-2 '(a b c)))
   (print (nreconc list-1 list-2))
   (print (equal list-1 '(1 2 3)))
   (print (equal list-2 '(a b c))))
@t{ |> } (3 2 1 A B C) 
@t{ |> } NIL
@t{ |> } T
@result{}  T

@end example

@subsubheading  Side Effects::

@b{revappend} does not modify either of its @i{arguments}.
@b{nreconc} is permitted to modify @i{list} but not @i{tail}.

Although it might be implemented differently,
@b{nreconc} is constrained to have side-effect behavior equivalent to:

@example
 (nconc (nreverse @i{list}) @i{tail})
@end example

@subsubheading  See Also::

@ref{reverse; nreverse}
,
@b{nreverse},
@ref{nconc}

@subsubheading  Notes::

The following functional equivalences are true, 
although good @i{implementations} will typically use a faster algorithm for
achieving the same effect:

@example
 (revappend @i{list} @i{tail}) @equiv{} (nconc (reverse @i{list}) @i{tail})
 (nreconc @i{list} @i{tail}) @equiv{} (nconc (nreverse @i{list}) @i{tail})
@end example

@node butlast, last, revappend, Conses Dictionary
@subsection butlast, nbutlast                                                [Function]

@code{butlast}  @i{list {&optional} n} @result{}  @i{result-list}

@code{nbutlast}  @i{list {&optional} n} @result{}  @i{result-list}

@subsubheading  Arguments and Values::

@i{list}---a @i{list}, 

	       which might be a @i{dotted list} but must not be a @i{circular list}.

@i{n}---a non-negative @i{integer}.

@i{result-list}---a @i{list}.

@subsubheading  Description::

@b{butlast} returns a copy of @i{list} from which the last
@i{n}

conses

have been omitted.
If @i{n} is not supplied, its value is 1.  
If there are fewer than @i{n} 

conses

in @i{list},
@b{nil} is returned and, in the case of @b{nbutlast},
@i{list} is not modified.  

@b{nbutlast} is like @b{butlast}, but @b{nbutlast} 
may modify @i{list}.
It changes the @i{cdr} of
the @i{cons} @i{n}+1 from the end of the @i{list} to @b{nil}.  

@subsubheading  Examples::
@example
 (setq lst '(1 2 3 4 5 6 7 8 9)) @result{}  (1 2 3 4 5 6 7 8 9)
 (butlast lst) @result{}  (1 2 3 4 5 6 7 8)
 (butlast lst 5) @result{}  (1 2 3 4)
 (butlast lst (+ 5 5)) @result{}  NIL
 lst @result{}  (1 2 3 4 5 6 7 8 9)
 (nbutlast lst 3) @result{}  (1 2 3 4 5 6)
 lst @result{}  (1 2 3 4 5 6)
 (nbutlast lst 99) @result{}  NIL
 lst @result{}  (1 2 3 4 5 6)
 (butlast '(a b c d)) @result{}  (A B C)
 (butlast '((a b) (c d))) @result{}  ((A B))
 (butlast '(a)) @result{}  NIL
 (butlast nil) @result{}  NIL
 (setq foo (list 'a 'b 'c 'd)) @result{}  (A B C D)
 (nbutlast foo) @result{}  (A B C)
 foo @result{}  (A B C)
 (nbutlast (list 'a)) @result{}  NIL
 (nbutlast '()) @result{}  NIL
@end example

@subsubheading  Exceptional Situations::

Should signal an error of @i{type} @b{type-error}
			      if @i{list} is not a @i{proper list} or a @i{dotted list}.

Should signal an error of @i{type} @b{type-error}
			      if @i{n} is not a non-negative @i{integer}.

@subsubheading  Notes::

@example
 (butlast @i{list} @i{n}) @equiv{} (ldiff @i{list} (last @i{list} @i{n}))
@end example

@node last, ldiff, butlast, Conses Dictionary
@subsection last                                                             [Function]

@code{last}  @i{list {&optional} n} @result{}  @i{tail}

@subsubheading  Arguments and Values::

@i{list}---a @i{list},

               which might be a @i{dotted list} but must not be a @i{circular list}.

@i{n}---a non-negative @i{integer}.
 The default is @t{1}.

@i{tail}---an @i{object}.

@subsubheading  Description::

@b{last} returns the last @i{n} @i{conses} 
 (not the last @i{n} elements) of @i{list}).
If @i{list} is @t{()}, @b{last} returns @t{()}.

If @i{n} is zero,
  the atom that terminates @i{list} is returned.
If @i{n} is greater than or equal to the number of @i{cons} cells in @i{list},
  the result is @i{list}.

@subsubheading  Examples::

@example
 (last nil) @result{}  NIL
 (last '(1 2 3)) @result{}  (3)
 (last '(1 2 . 3)) @result{}  (2 . 3)
 (setq x (list 'a 'b 'c 'd)) @result{}  (A B C D)
 (last x) @result{}  (D)
 (rplacd (last x) (list 'e 'f)) x @result{}  (A B C D E F)
 (last x) @result{}  (F)

 (last '(a b c))   @result{}  (C)

 (last '(a b c) 0) @result{}  ()
 (last '(a b c) 1) @result{}  (C)
 (last '(a b c) 2) @result{}  (B C)
 (last '(a b c) 3) @result{}  (A B C)
 (last '(a b c) 4) @result{}  (A B C)

 (last '(a . b) 0) @result{}  B
 (last '(a . b) 1) @result{}  (A . B)
 (last '(a . b) 2) @result{}  (A . B)
@end example

@subsubheading  Exceptional Situations::

The consequences are undefined if @i{list} is a @i{circular list}.

Should signal an error of @i{type} @b{type-error}
			      if @i{n} is not a non-negative @i{integer}.

@subsubheading  See Also::

@ref{butlast; nbutlast}
,
@ref{nth}

@subsubheading  Notes::                                             

The following code could be used to define @b{last}.

@example
 (defun last (list &optional (n 1))
   (check-type n (integer 0))
   (do ((l list (cdr l))
        (r list)
        (i 0 (+ i 1)))
       ((atom l) r)
     (if (>= i n) (pop r))))
@end example

@node ldiff, nthcdr, last, Conses Dictionary
@subsection ldiff, tailp                                                     [Function]

@code{ldiff}  @i{list object} @result{}  @i{result-list}

@code{tailp}  @i{object list} @result{}  @i{generalized-boolean}

@subsubheading  Arguments and Values::

@i{list}---a @i{list},

                   which might be a @i{dotted list}.

@i{object}---an @i{object}.

@i{result-list}---a @i{list}.

@i{generalized-boolean}---a @i{generalized boolean}.

@subsubheading  Description::

If @i{object} is the @i{same} as some @i{tail} of @i{list},
@b{tailp} returns @i{true};
otherwise, it returns @i{false}.

If @i{object} is the @i{same} as some @i{tail} of @i{list},
@b{ldiff} returns a @i{fresh} @i{list} 
of the @i{elements} of @i{list} 
that precede @b{object} in the @i{list structure} of @i{list};
otherwise, it returns a @i{copy}_2 of @i{list}.

@subsubheading  Examples::

@example
 (let ((lists '#((a b c) (a b c . d))))
   (dotimes (i (length lists)) ()
     (let ((list (aref lists i)))
       (format t "~2&list=~S ~21T(tailp object list)~
                  ~44T(ldiff list object)~
         (let ((objects (vector list (cddr list) (copy-list (cddr list))
                                '(f g h) '() 'd 'x)))
           (dotimes (j (length objects)) ()
             (let ((object (aref objects j)))
               (format t "~& object=~S ~21T~S ~44T~S"
                       object (tailp object list) (ldiff list object))))))))
@t{ |> } 
@t{ |> } list=(A B C)         (tailp object list)    (ldiff list object)
@t{ |> }  object=(A B C)      T                      NIL
@t{ |> }  object=(C)          T                      (A B)
@t{ |> }  object=(C)          NIL                    (A B C)
@t{ |> }  object=(F G H)      NIL                    (A B C)
@t{ |> }  object=NIL          T                      (A B C)
@t{ |> }  object=D            NIL                    (A B C)
@t{ |> }  object=X            NIL                    (A B C)
@t{ |> } 
@t{ |> } list=(A B C . D)     (tailp object list)    (ldiff list object)
@t{ |> }  object=(A B C . D)  T                      NIL
@t{ |> }  object=(C . D)      T                      (A B)
@t{ |> }  object=(C . D)      NIL                    (A B C . D)
@t{ |> }  object=(F G H)      NIL                    (A B C . D)
@t{ |> }  object=NIL          NIL                    (A B C . D)
@t{ |> }  object=D            T                      (A B C)
@t{ |> }  object=X            NIL                    (A B C . D)
@result{}  NIL
@end example

@subsubheading  Side Effects::

Neither @b{ldiff} nor @b{tailp} modifies either of its @i{arguments}.

@subsubheading  Exceptional Situations::

Should be prepared to signal an error of @i{type} @b{type-error}
			 if @i{list} is not a @i{proper list} or a @i{dotted list}.

@subsubheading  See Also::

@ref{set-difference; nset-difference}

@subsubheading  Notes::

If the @i{list} is a @i{circular list},
@b{tailp} will reliably @i{yield} a @i{value} 
only if the given @i{object} is in fact a @i{tail} of @i{list}.
Otherwise, the consequences are unspecified:
a given @i{implementation} which detects the circularity must return @i{false},
but since an @i{implementation} is not obliged to detect such a @i{situation},
@b{tailp} might just loop indefinitely without returning in that case.

@b{tailp} could be defined as follows:

@example
 (defun tailp (object list)
   (do ((list list (cdr list)))
       ((atom list) (eql list object))
      (if (eql object list)
          (return t))))
@end example

and @b{ldiff} could be defined by:

@example
(defun ldiff (list object)
  (do ((list list (cdr list))
       (r '() (cons (car list) r)))
      ((atom list)
       (if (eql list object) (nreverse r) (nreconc r list)))
    (when (eql object list)
      (return (nreverse r)))))
@end example

@node nthcdr, rest, ldiff, Conses Dictionary
@subsection nthcdr                                                           [Function]

@code{nthcdr}  @i{n list} @result{}  @i{tail}

@subsubheading  Arguments and Values::

@i{n}---a non-negative @i{integer}.

@i{list}---a @i{list},

	       which might be a @i{dotted list} or a @i{circular list}.

@i{tail}---an @i{object}.

@subsubheading  Description::

Returns the @i{tail} of @i{list} that would be obtained by calling @b{cdr}
@i{n} times in succession.

@subsubheading  Examples::

@example
 (nthcdr 0 '()) @result{}  NIL
 (nthcdr 3 '()) @result{}  NIL
 (nthcdr 0 '(a b c)) @result{}  (A B C)
 (nthcdr 2 '(a b c)) @result{}  (C)
 (nthcdr 4 '(a b c)) @result{}  ()
 (nthcdr 1 '(0 . 1)) @result{}  1

 (locally (declare (optimize (safety 3)))
   (nthcdr 3 '(0 . 1)))
 Error: Attempted to take CDR of 1.
@end example

@subsubheading  Exceptional Situations::

Should signal an error of @i{type} @b{type-error}
			      if @i{n} is not a non-negative @i{integer}.

For @i{n} being an integer greater than @t{1},
the error checking done by @t{(nthcdr @i{n} @i{list})}
is the same as for @t{(nthcdr (- @i{n} 1) (cdr @i{list}))};
see the @i{function} @b{cdr}.

@subsubheading  See Also::

@b{cdr},
@ref{nth}
,
@ref{rest}

@node rest, member, nthcdr, Conses Dictionary
@subsection rest                                                             [Accessor]

@code{rest}  @i{list} @result{}  @i{tail}

(setf (@code{         rest} @i{list}) new-tail)@*

@subsubheading  Arguments and Values::

@i{list}---a @i{list},

	       which might be a @i{dotted list} or a @i{circular list}.

@i{tail}---an @i{object}.

@subsubheading  Description::

@b{rest} performs the same operation as @b{cdr},
but mnemonically complements @b{first}.
Specifically,

@example
 (rest @i{list}) @equiv{} (cdr @i{list})
 (setf (rest @i{list}) @i{new-tail}) @equiv{} (setf (cdr @i{list}) @i{new-tail})
@end example

@subsubheading  Examples::

@example
 (rest '(1 2)) @result{}  (2)
 (rest '(1 . 2)) @result{}  2
 (rest '(1)) @result{}  NIL
 (setq *cons* '(1 . 2)) @result{}  (1 . 2)
 (setf (rest *cons*) "two") @result{}  "two"
 *cons* @result{}  (1 . "two")
@end example

@subsubheading  See Also::

@b{cdr},
@ref{nthcdr}

@subsubheading  Notes::

@b{rest} is often preferred stylistically over @b{cdr}
when the argument is to being subjectively viewed as a @i{list} 
rather than as a @i{cons}.

@node member, mapc, rest, Conses Dictionary
@subsection member, member-if, member-if-not                                 [Function]

@code{member}  @i{item      list {&key} key test test-not} @result{}  @i{tail}

@code{member-if}  @i{predicate list {&key} key} @result{}  @i{tail}

@code{member-if-not}  @i{predicate list {&key} key} @result{}  @i{tail}

@subsubheading  Arguments and Values:: 

@i{item}---an @i{object}.

@i{list}---a @i{proper list}.

@i{predicate}---a @i{designator} for 
		    a @i{function} of one @i{argument}
		    that returns a @i{generalized boolean}.

@i{test}---a @i{designator} for a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{test-not}---a @i{designator} for 
  a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{key}---a @i{designator} for a @i{function} of one argument,
  or @b{nil}.

@i{tail}---a @i{list}.

@subsubheading  Description::

@b{member}, @b{member-if}, and @b{member-if-not} each
search @i{list} for @i{item} or for a top-level element that 
@i{satisfies the test}.  The argument to the @i{predicate} function 
is an element of @i{list}.

If some element @i{satisfies the test},
the tail of @i{list} beginning
with this element is returned; otherwise @b{nil} is returned.

@i{list} is searched on the top level only. 

@subsubheading  Examples::

@example
 (member 2 '(1 2 3)) @result{}  (2 3)                                 
 (member 2 '((1 . 2) (3 . 4)) :test-not #'= :key #'cdr) @result{}  ((3 . 4))
 (member 'e '(a b c d)) @result{}  NIL
@end example

@example
 (member-if #'listp '(a b nil c d)) @result{}  (NIL C D)
 (member-if #'numberp '(a #\Space 5/3 foo)) @result{}  (5/3 FOO)
 (member-if-not #'zerop 
                 '(3 6 9 11 . 12)
                 :key #'(lambda (x) (mod x 3))) @result{}  (11 . 12)
@end example

@subsubheading  Exceptional Situations::

Should be prepared to signal an error of @i{type} @b{type-error}
			 if @i{list} is not a @i{proper list}.

@subsubheading  See Also::

@ref{find; find-if; find-if-not}
,
@ref{position; position-if; position-if-not}
,

@ref{Traversal Rules and Side Effects}

@subsubheading  Notes::

The @t{:test-not} parameter is deprecated.

The @i{function} @b{member-if-not} is deprecated.

In the following

@example
 (member 'a '(g (a y) c a d e a f)) @result{}  (A D E A F)
@end example

the value returned by @b{member} is @i{identical} to the portion
of the @i{list} beginning with @t{a}.  Thus @b{rplaca} on the
result of @b{member} can be used to alter the part of the @i{list}
where @t{a} was found (assuming a check has been made that @b{member}
did not return @b{nil}).

@node mapc, acons, member, Conses Dictionary
@subsection mapc, mapcar, mapcan, mapl, maplist, mapcon                      [Function]

@code{mapc}  @i{function {&rest} lists^+} @result{}  @i{list-1}

@code{mapcar}  @i{function {&rest} lists^+} @result{}  @i{result-list}

@code{mapcan}  @i{function {&rest} lists^+} @result{}  @i{concatenated-results}

@code{mapl}  @i{function {&rest} lists^+} @result{}  @i{list-1}

@code{maplist}  @i{function {&rest} lists^+} @result{}  @i{result-list}

@code{mapcon}  @i{function {&rest} lists^+} @result{}  @i{concatenated-results}

@subsubheading  Arguments and Values:: 

@i{function}---a @i{designator} for a @i{function} 
  that must take as many @i{arguments} as there are @i{lists}.

@i{list}---a @i{proper list}.

@i{list-1}---the first @i{list} (which must be a @i{proper list}).

@i{result-list}---a @i{list}.

@i{concatenated-results}---a @i{list}.

@subsubheading  Description::

The mapping operation involves applying @i{function} to
successive sets of arguments in which
one argument is obtained from each @i{sequence}.
Except for @b{mapc} and @b{mapl},
the result contains the results returned by @i{function}.
In the cases of @b{mapc} and @b{mapl},
the resulting @i{sequence} is @i{list}.

@i{function} is called
first on all the elements with index @t{0}, then on all those
with index @t{1}, and so on.
@i{result-type} specifies the @i{type} of
the 
resulting @i{sequence}.

If @i{function} is a @i{symbol}, it is @b{coerce}d
to a @i{function} as if by @b{symbol-function}.

@b{mapcar} operates on successive @i{elements} of the @i{lists}.
@i{function} is applied to the first @i{element} of each @i{list},
then to the second @i{element} of each @i{list}, and so on.
The iteration terminates when the shortest @i{list} runs out,
and excess elements in other lists are ignored.
The value returned by @b{mapcar} is a @i{list}
of the results of successive calls to @i{function}.

@b{mapc} is like @b{mapcar} except that the results of 
applying @i{function} are not accumulated.
The @i{list} argument is returned.

@b{maplist} is like @b{mapcar} except that
@i{function} is applied to successive sublists of the @i{lists}.
@i{function} 
is first applied to the @i{lists} themselves, 
and then to the @i{cdr} of each
@i{list}, and then to the @i{cdr} of the @i{cdr}
of each @i{list}, and so on.  

@b{mapl} is like @b{maplist} except that the results of 
applying @i{function} are not accumulated;
@i{list-1} is returned.

@b{mapcan} and @b{mapcon} are like @b{mapcar} and
@b{maplist} respectively, except that the results of
applying @i{function} are combined 
into a @i{list} by the use of @b{nconc}
rather than @b{list}.
That is,

@example
 (mapcon f x1 ... xn)
   @equiv{} (apply #'nconc (maplist f x1 ... xn))
@end example

and similarly for the relationship between @b{mapcan} 
and @b{mapcar}.

@subsubheading  Examples::

@example
 (mapcar #'car '((1 a) (2 b) (3 c))) @result{}  (1 2 3) 
 (mapcar #'abs '(3 -4 2 -5 -6)) @result{}  (3 4 2 5 6)
 (mapcar #'cons '(a b c) '(1 2 3)) @result{}  ((A . 1) (B . 2) (C . 3))

 (maplist #'append '(1 2 3 4) '(1 2) '(1 2 3)) 
@result{}  ((1 2 3 4 1 2 1 2 3) (2 3 4 2 2 3)) 
 (maplist #'(lambda (x) (cons 'foo x)) '(a b c d))
@result{}  ((FOO A B C D) (FOO B C D) (FOO C D) (FOO D))
 (maplist #'(lambda (x) (if (member (car x) (cdr x)) 0 1)) '(a b a c d b c))
@result{}  (0 0 1 0 1 1 1)
;An entry is 1 if the corresponding element of the input
;  list was the last instance of that element in the input list.

 (setq dummy nil) @result{}  NIL 
 (mapc #'(lambda (&rest x) (setq dummy (append dummy x)))
        '(1 2 3 4)
        '(a b c d e)
        '(x y z)) @result{}  (1 2 3 4) 
 dummy @result{}  (1 A X 2 B Y 3 C Z)                   

 (setq dummy nil) @result{}  NIL 
 (mapl #'(lambda (x) (push x dummy)) '(1 2 3 4)) @result{}  (1 2 3 4) 
 dummy @result{}  ((4) (3 4) (2 3 4) (1 2 3 4)) 

 (mapcan #'(lambda (x y) (if (null x) nil (list x y)))
          '(nil nil nil d e)
          '(1 2 3 4 5 6)) @result{}  (D 4 E 5) 
 (mapcan #'(lambda (x) (and (numberp x) (list x)))
          '(a 1 b c 3 4 d 5))
@result{}  (1 3 4 5)
@end example

In this case the function serves as a filter; 
this is a standard @r{Lisp} idiom using @b{mapcan}.

@example
 (mapcon #'list '(1 2 3 4)) @result{}  ((1 2 3 4) (2 3 4) (3 4) (4)) 
@end example

@subsubheading  Exceptional Situations::

Should be prepared to signal an error of @i{type} @b{type-error}
			    if any @i{list} is not a @i{proper list}.

@subsubheading  See Also::

@ref{dolist}
,
@ref{map}
,

@ref{Traversal Rules and Side Effects}

@node acons, assoc, mapc, Conses Dictionary
@subsection acons                                                            [Function]

@code{acons}  @i{key datum alist} @result{}  @i{new-alist}

@subsubheading  Arguments and Values:: 

@i{key}---an @i{object}.

@i{datum}---an @i{object}.

@i{alist}---an @i{association list}.

@i{new-alist}---an @i{association list}.

@subsubheading  Description::

Creates a @i{fresh} @i{cons},
the @i{cdr} of which is @i{alist} and
the @i{car} of which is another @i{fresh} @i{cons},
 the @i{car} of which is @i{key} and
 the @i{cdr} of which is @i{datum}.

@subsubheading  Examples::

@example
 (setq alist '()) @result{}  NIL
 (acons 1 "one" alist) @result{}  ((1 . "one"))
 alist @result{}  NIL
 (setq alist (acons 1 "one" (acons 2 "two" alist))) @result{}  ((1 . "one") (2 . "two"))
 (assoc 1 alist) @result{}  (1 . "one")
 (setq alist (acons 1 "uno" alist)) @result{}  ((1 . "uno") (1 . "one") (2 . "two"))
 (assoc 1 alist) @result{}  (1 . "uno")
@end example

@subsubheading  See Also::

@ref{assoc; assoc-if; assoc-if-not}
, 
@ref{pairlis}

@subsubheading  Notes::

@example
(acons @i{key} @i{datum} @i{alist}) @equiv{} (cons (cons @i{key} @i{datum}) @i{alist})
@end example

@node assoc, copy-alist, acons, Conses Dictionary
@subsection assoc, assoc-if, assoc-if-not                                    [Function]

@code{assoc}  @i{item      alist {&key} key test test-not} @result{}  @i{entry}

@code{assoc-if}  @i{predicate alist {&key} key} @result{}  @i{entry}

@code{assoc-if-not}  @i{predicate alist {&key} key} @result{}  @i{entry}

@subsubheading  Arguments and Values::

@i{item}---an @i{object}.

@i{alist}---an @i{association list}.

@i{predicate}---a @i{designator} for 
		    a @i{function} of one @i{argument}
		    that returns a @i{generalized boolean}.

@i{test}---a @i{designator} for a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{test-not}---a @i{designator} for 
  a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{key}---a @i{designator} for a @i{function} of one argument,
  or @b{nil}.

@i{entry}---a @i{cons} that is an @i{element} of @i{alist},
	        or @b{nil}.

@subsubheading  Description::

@b{assoc}, @b{assoc-if}, and @b{assoc-if-not}
return the first @i{cons} in @i{alist} whose @i{car} @i{satisfies the test},
or @b{nil} if no such @i{cons} is found.

For @b{assoc}, @b{assoc-if}, and @b{assoc-if-not}, if @b{nil} appears
in @i{alist} in place of a pair, it is ignored.

@subsubheading  Examples::

@example
 (setq values '((x . 100) (y . 200) (z . 50))) @result{}  ((X . 100) (Y . 200) (Z . 50))
 (assoc 'y values) @result{}  (Y . 200)
 (rplacd (assoc 'y values) 201) @result{}  (Y . 201)
 (assoc 'y values) @result{}  (Y . 201)
 (setq alist '((1 . "one")(2 . "two")(3 . "three"))) 
@result{}  ((1 . "one") (2 . "two") (3 . "three"))
 (assoc 2 alist) @result{}  (2 . "two")
 (assoc-if #'evenp alist) @result{}  (2 . "two")
 (assoc-if-not #'(lambda(x) (< x 3)) alist) @result{}  (3 . "three")
 (setq alist '(("one" . 1)("two" . 2))) @result{}  (("one" . 1) ("two" . 2))
 (assoc "one" alist) @result{}  NIL
 (assoc "one" alist :test #'equalp) @result{}  ("one" . 1)
 (assoc "two" alist :key #'(lambda(x) (char x 2))) @result{}  NIL 
 (assoc #\o alist :key #'(lambda(x) (char x 2))) @result{}  ("two" . 2)
 (assoc 'r '((a . b) (c . d) (r . x) (s . y) (r . z))) @result{}   (R . X)
 (assoc 'goo '((foo . bar) (zoo . goo))) @result{}  NIL
 (assoc '2 '((1 a b c) (2 b c d) (-7 x y z))) @result{}  (2 B C D)
 (setq alist '(("one" . 1) ("2" . 2) ("three" . 3)))
@result{}  (("one" . 1) ("2" . 2) ("three" . 3))
 (assoc-if-not #'alpha-char-p alist
               :key #'(lambda (x) (char x 0))) @result{}  ("2" . 2)
@end example

@subsubheading  Exceptional Situations::

Should be prepared to signal an error of @i{type} @b{type-error}
			 if @i{alist} is not an @i{association list}.

@subsubheading  See Also::

@ref{rassoc; rassoc-if; rassoc-if-not}
,
@ref{find; find-if; find-if-not}
,
@ref{member; member-if; member-if-not}
,
@ref{position; position-if; position-if-not}
,

@ref{Traversal Rules and Side Effects}

@subsubheading  Notes::

The @t{:test-not} parameter is deprecated.

The @i{function} @b{assoc-if-not} is deprecated.

It is possible to @b{rplacd} the result of @b{assoc}, provided
that it is not @b{nil},
in order to ``update'' @i{alist}. 

The two expressions

@example
 (assoc item list :test fn)
@end example

and

@example
 (find item list :test fn :key #'car)
@end example

are equivalent in meaning with one exception:
if @b{nil} appears in @i{alist} in place of a pair,
and @i{item} is @b{nil},
@b{find} will compute the @i{car} of the @b{nil} in @i{alist},
find that it is equal to @i{item}, and return @b{nil},
whereas @b{assoc} will ignore the @b{nil} in @i{alist} and continue
to search for an actual @i{cons} whose @i{car} is @b{nil}.

@node copy-alist, pairlis, assoc, Conses Dictionary
@subsection copy-alist                                                       [Function]

@code{copy-alist}  @i{alist} @result{}  @i{new-alist}

@subsubheading  Arguments and Values::

@i{alist}---an @i{association list}.

@i{new-alist}---an @i{association list}.

@subsubheading  Description::

@b{copy-alist} returns a @i{copy} of @i{alist}.

The @i{list structure} of @i{alist} is copied,
and the @i{elements} of @i{alist} which are @i{conses} are
also copied (as @i{conses} only). 
Any other @i{objects} which are referred to, 
whether directly or indirectly,
by the @i{alist} continue to be shared.

@subsubheading  Examples::

@example
(defparameter *alist* (acons 1 "one" (acons 2 "two" '())))
*alist* @result{}  ((1 . "one") (2 . "two"))
(defparameter *list-copy* (copy-list *alist*))
*list-copy* @result{}  ((1 . "one") (2 . "two"))
(defparameter *alist-copy* (copy-alist *alist*))
*alist-copy* @result{}  ((1 . "one") (2 . "two"))
(setf (cdr (assoc 2 *alist-copy*)) "deux") @result{}  "deux"
*alist-copy* @result{}  ((1 . "one") (2 . "deux"))
*alist* @result{}  ((1 . "one") (2 . "two"))
(setf (cdr (assoc 1 *list-copy*)) "uno") @result{}  "uno"
*list-copy* @result{}  ((1 . "uno") (2 . "two"))
*alist* @result{}  ((1 . "uno") (2 . "two"))
@end example

@subsubheading  See Also::

@ref{copy-list}

@node pairlis, rassoc, copy-alist, Conses Dictionary
@subsection pairlis                                                          [Function]

@code{pairlis}  @i{keys data {&optional} alist} @result{}  @i{new-alist}

@subsubheading  Arguments and Values::

@i{keys}---a @i{proper list}.

@i{data}---a @i{proper list}.

@i{alist}---an @i{association list}.
  The default is the @i{empty list}.

@i{new-alist}---an @i{association list}.

@subsubheading  Description::

Returns an @i{association list} that associates
elements of @i{keys} to corresponding elements of @i{data}.
The consequences are undefined if @i{keys} and @i{data} are 
not of the same @i{length}.  

If @i{alist} is supplied, @b{pairlis} returns
a modified @i{alist} with the
new pairs prepended to it.
The new pairs may appear in the resulting @i{association list} in 
either forward or backward order.
The result of 

@example
 (pairlis '(one two) '(1 2) '((three . 3) (four . 19)))
@end example

might be

@example
 ((one . 1) (two . 2) (three . 3) (four . 19))
@end example

or

@example
 ((two . 2) (one . 1) (three . 3) (four . 19))
@end example

@subsubheading  Examples::
@example
 (setq keys '(1 2 3)
        data '("one" "two" "three")
        alist '((4 . "four"))) @result{}  ((4 . "four"))
 (pairlis keys data) @result{}  ((3 . "three") (2 . "two") (1 . "one"))
 (pairlis keys data alist)
@result{}  ((3 . "three") (2 . "two") (1 . "one") (4 . "four"))
 alist @result{}  ((4 . "four"))
@end example

@subsubheading  Exceptional Situations::

Should be prepared to signal an error of @i{type} @b{type-error}
			 if @i{keys} and @i{data} are not @i{proper lists}.

@subsubheading  See Also::

@ref{acons}

@node rassoc, get-properties, pairlis, Conses Dictionary
@subsection rassoc, rassoc-if, rassoc-if-not                                 [Function]

@code{rassoc}  @i{item      alist  {&key} key test test-not} @result{}  @i{entry}

@code{rassoc-if}  @i{predicate alist {&key} key} @result{}  @i{entry}

@code{rassoc-if-not}  @i{predicate alist {&key} key} @result{}  @i{entry}

@subsubheading  Arguments and Values:: 

@i{item}---an @i{object}.

@i{alist}---an @i{association list}.

@i{predicate}---a @i{designator} for
		    a @i{function} of one @i{argument}
		    that returns a @i{generalized boolean}.

@i{test}---a @i{designator} for a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{test-not}---a @i{designator} for 
  a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{key}---a @i{designator} for a @i{function} of one argument,
  or @b{nil}.

@i{entry}---a @i{cons} that is an @i{element} of the @i{alist},
		or @b{nil}.

@subsubheading  Description::

@b{rassoc}, @b{rassoc-if}, and @b{rassoc-if-not}
return the first @i{cons} whose @i{cdr} 
@i{satisfies the test}.
If no such @i{cons} is found, @b{nil}
is returned.

If @b{nil} appears in @i{alist} in place of a pair, it is  ignored.

@subsubheading  Examples::

@example
 (setq alist '((1 . "one") (2 . "two") (3 . 3))) 
@result{}  ((1 . "one") (2 . "two") (3 . 3))
 (rassoc 3 alist) @result{}  (3 . 3)
 (rassoc "two" alist) @result{}  NIL
 (rassoc "two" alist :test 'equal) @result{}  (2 . "two")
 (rassoc 1 alist :key #'(lambda (x) (if (numberp x) (/ x 3)))) @result{}  (3 . 3)
 (rassoc 'a '((a . b) (b . c) (c . a) (z . a))) @result{}  (C . A)
 (rassoc-if #'stringp alist) @result{}  (1 . "one")
 (rassoc-if-not #'vectorp alist) @result{}  (3 . 3)
@end example

@subsubheading  See Also::

@ref{assoc; assoc-if; assoc-if-not}
,

@ref{Traversal Rules and Side Effects}

@subsubheading  Notes::

The @t{:test-not} parameter is deprecated.

The @i{function} @b{rassoc-if-not} is deprecated.

It is possible to @b{rplaca} the result of @b{rassoc}, 
provided that it is not @b{nil}, in order to ``update'' @i{alist}. 

The expressions

@example
 (rassoc item list :test fn)
@end example

and

@example
 (find item list :test fn :key #'cdr)
@end example

are equivalent in meaning, except when the @t{item} is @b{nil}
and @b{nil} appears in place of a pair in the @i{alist}.
See the @i{function} @b{assoc}. 

@node get-properties, getf, rassoc, Conses Dictionary
@subsection get-properties                                                   [Function]

@code{get-properties}  @i{plist indicator-list} @result{}  @i{indicator, value, tail}

@subsubheading  Arguments and Values:: 

@i{plist}---a @i{property list}.

@i{indicator-list}---a @i{proper list} (of @i{indicators}).

@i{indicator}---an @i{object} that is an @i{element} of @i{indicator-list}.

@i{value}---an @i{object}.

@i{tail}---a @i{list}.

@subsubheading  Description::

@b{get-properties} is used to look up any of several
@i{property list} entries all at once.

It searches the @i{plist} for the first entry whose @i{indicator} 
is @i{identical} to one of the @i{objects} in @i{indicator-list}.
If such an entry is found, the @i{indicator} and @i{value} returned
are the @i{property indicator} and its associated @i{property value},
and the @i{tail} returned is the @i{tail} of the @i{plist}
that begins with the found entry (@i{i.e.}, whose @i{car} is the @i{indicator}).
If no such entry is found, the @i{indicator}, @i{value}, and @i{tail} 
are all @b{nil}.

@subsubheading  Examples::

@example
 (setq x '()) @result{}  NIL
 (setq *indicator-list* '(prop1 prop2)) @result{}  (PROP1 PROP2)
 (getf x 'prop1) @result{}  NIL
 (setf (getf x 'prop1) 'val1) @result{}  VAL1
 (eq (getf x 'prop1) 'val1) @result{}  @i{true}
 (get-properties x *indicator-list*) @result{}  PROP1, VAL1, (PROP1 VAL1)
 x @result{}  (PROP1 VAL1)
@end example

@subsubheading  See Also::

@ref{get}
, 
@ref{getf}

@node getf, remf, get-properties, Conses Dictionary
@subsection getf                                                             [Accessor]

@code{getf}  @i{plist indicator {&optional} default} @result{}  @i{value}

(setf (@code{         getf} @i{place indicator {&optional} default}) new-value)@*

@subsubheading  Arguments and Values::

@i{plist}---a @i{property list}.

@i{place}---a @i{place}, the @i{value} of which is a @i{property list}.

@i{indicator}---an @i{object}.

@i{default}---an @i{object}.
 The default is @b{nil}.

@i{value}---an @i{object}.

@i{new-value}---an @i{object}.

@subsubheading  Description::

@b{getf} finds a @i{property} on the @i{plist}
whose @i{property indicator} is @i{identical} to @i{indicator},
and returns its corresponding @i{property value}.

If there are multiple @i{properties}_1 with that @i{property indicator},
@b{getf} uses the first such @i{property}.

If there is no @i{property} with that @i{property indicator},
@i{default} is returned.

@b{setf} of @b{getf} may be used to associate a new @i{object}
with an existing indicator in the @i{property list} held by @i{place},
or to create a new assocation if none exists.

If there are multiple @i{properties}_1 with that @i{property indicator},
@b{setf} of @b{getf} associates the @i{new-value} 
with the first such @i{property}.

When a @b{getf} @i{form} is used as a @b{setf} @i{place},
any @i{default} which is supplied is evaluated according to normal
left-to-right evaluation rules, but its @i{value} is ignored.

@b{setf} of @b{getf} is permitted to either
    @i{write} the @i{value} of @i{place} itself,
 or modify of any part, @i{car} or @i{cdr}, 
     of the @i{list structure} held by @i{place}.

@subsubheading  Examples::

@example
 (setq x '()) @result{}  NIL
 (getf x 'prop1) @result{}  NIL
 (getf x 'prop1 7) @result{}  7
 (getf x 'prop1) @result{}  NIL
 (setf (getf x 'prop1) 'val1) @result{}  VAL1
 (eq (getf x 'prop1) 'val1) @result{}  @i{true}
 (getf x 'prop1) @result{}  VAL1
 (getf x 'prop1 7) @result{}  VAL1
 x @result{}  (PROP1 VAL1)

;; Examples of implementation variation permitted.
 (setq foo (list 'a 'b 'c 'd 'e 'f)) @result{}  (A B C D E F)
 (setq bar (cddr foo)) @result{}  (C D E F)
 (remf foo 'c) @result{}  @i{true}
 foo @result{}  (A B E F)
 bar
@result{}  (C D E F)
@i{OR}@result{} (C)
@i{OR}@result{} (NIL)
@i{OR}@result{} (C NIL)
@i{OR}@result{} (C D)
@end example

@subsubheading  See Also::

@ref{get}
,
@ref{get-properties}
,
@ref{setf; psetf}
,
@ref{Function Call Forms as Places}

@subsubheading  Notes::

There is no way (using @b{getf}) to distinguish an absent property
from one whose value is @i{default}; but see @b{get-properties}.

Note that while supplying a @i{default} argument to @b{getf}
in a @b{setf} situation is sometimes not very interesting,
it is still important because some macros, such as @b{push} and
@b{incf}, require a @i{place} argument which data is both @i{read}
from and @i{written} to.  In such a context, if a @i{default} 
argument is to be supplied for the @i{read} situation, it must be
syntactically valid for the @i{write} situation as well. For example,

@example
 (let ((plist '()))
   (incf (getf plist 'count 0))
   plist) @result{}  (COUNT 1)
@end example

@node remf, intersection, getf, Conses Dictionary
@subsection remf                                                                [Macro]

@code{remf}  @i{place indicator} @result{}  @i{generalized-boolean}

@subsubheading  Arguments and Values::

@i{place}---a @i{place}.

@i{indicator}---an @i{object}.

@i{generalized-boolean}---a @i{generalized boolean}.

@subsubheading  Description::

@b{remf} removes from the @i{property list} stored in @i{place}
a @i{property}_1 with a @i{property indicator}
@i{identical} to @i{indicator}.

If there are multiple @i{properties}_1 with the @i{identical} key,
@b{remf} only removes the first such @i{property}.

@b{remf} returns @i{false} if no such @i{property} was found,
or @i{true} if a property was found.

The @i{property indicator} 
and the corresponding @i{property value} 
are removed in an undefined order
by destructively splicing the property list.  

@b{remf} is permitted to either @b{setf} @i{place} or to 
@b{setf} any part, @b{car} or @b{cdr}, 
of the @i{list structure} held by that @i{place}.

For information about the @i{evaluation} of @i{subforms} of @i{place},
see @ref{Evaluation of Subforms to Places}.

@subsubheading  Examples::

@example
 (setq x (cons () ())) @result{}  (NIL)
 (setf (getf (car x) 'prop1) 'val1) @result{}  VAL1
 (remf (car x) 'prop1) @result{}  @i{true}
 (remf (car x) 'prop1) @result{}  @i{false}
@end example

@subsubheading  Side Effects::

The property list stored in @i{place} is modified.

@subsubheading  See Also::

@ref{remprop}
, 
@ref{getf}

@node intersection, adjoin, remf, Conses Dictionary
@subsection intersection, nintersection                                      [Function]

@code{intersection}  @i{list-1 list-2 {&key} key test test-not} @result{}  @i{result-list}

@code{nintersection}  @i{list-1 list-2 {&key} key test test-not} @result{}  @i{result-list}

@subsubheading  Arguments and Values:: 

@i{list-1}---a @i{proper list}.

@i{list-2}---a @i{proper list}.

@i{test}---a @i{designator} for a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{test-not}---a @i{designator} for 
  a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{key}---a @i{designator} for a @i{function} of one argument,
  or @b{nil}.

@i{result-list}---a @i{list}.

@subsubheading  Description::

@b{intersection} and @b{nintersection} return a @i{list}
that contains every element that occurs in both @i{list-1} and @i{list-2}.

@b{nintersection} is the destructive version of @b{intersection}.
It performs the same operation,
but may destroy @i{list-1} using its cells to construct the result.

@i{list-2} is not destroyed.

The intersection operation is described as follows.
For all possible ordered pairs consisting of 
    one @i{element} from @i{list-1} 
and one @i{element} from @i{list-2},
@t{:test} or @t{:test-not} are used 
to determine whether they @i{satisfy the test}.  
The first argument to the @t{:test} or @t{:test-not}
function is an element of @i{list-1}; the second argument is an
element of @i{list-2}.
If @t{:test} or @t{:test-not} is not supplied, @b{eql}
is used.
It is an error if @t{:test} and @t{:test-not} are supplied in
the same function call.

If @t{:key} is supplied (and not @b{nil}), it is used to
extract the part to be tested from the @i{list} element. 
The argument to the @t{:key} function
is an element of either @i{list-1} or @i{list-2};
the @t{:key} function typically returns part of the supplied element.
If @t{:key} is not supplied or @b{nil}, the @i{list-1} and
@i{list-2} elements are used.

For every pair that @i{satifies the test},
exactly one of the two elements of the pair will be put in the result.
No element from either @i{list} appears in the result that does not 
@i{satisfy the test} for
an element from the other @i{list}.
If one of the @i{lists} contains duplicate
elements, there may be duplication in the result.

There is no guarantee that the order of elements in the result will
reflect the ordering of the arguments in any particular way.
The result @i{list} may share cells with, 
or be @b{eq} to, either @i{list-1} or @i{list-2}
if appropriate.

@subsubheading  Examples::

@example
 (setq list1 (list 1 1 2 3 4 a b c "A" "B" "C" "d")
       list2 (list 1 4 5 b c d "a" "B" "c" "D")) 
  @result{}  (1 4 5 B C D "a" "B" "c" "D")
 (intersection list1 list2) @result{}  (C B 4 1 1)
 (intersection list1 list2 :test 'equal) @result{}  ("B" C B 4 1 1)
 (intersection list1 list2 :test #'equalp) @result{}  ("d" "C" "B" "A" C B 4 1 1) 
 (nintersection list1 list2) @result{}  (1 1 4 B C)
 list1 @result{}  @i{implementation-dependent} ;@i{e.g.}, (1 1 4 B C)
 list2 @result{}  @i{implementation-dependent} ;@i{e.g.}, (1 4 5 B C D "a" "B" "c" "D")
 (setq list1 (copy-list '((1 . 2) (2 . 3) (3 . 4) (4 . 5))))
@result{}  ((1 . 2) (2 . 3) (3 . 4) (4 . 5)) 
 (setq list2 (copy-list '((1 . 3) (2 . 4) (3 . 6) (4 . 8))))
@result{}  ((1 . 3) (2 . 4) (3 . 6) (4 . 8)) 
 (nintersection list1 list2 :key #'cdr) @result{}  ((2 . 3) (3 . 4)) 
 list1 @result{}  @i{implementation-dependent} ;@i{e.g.}, ((1 . 2) (2 . 3) (3 . 4)) 
 list2 @result{}  @i{implementation-dependent} ;@i{e.g.}, ((1 . 3) (2 . 4) (3 . 6) (4 . 8)) 
@end example

@subsubheading  Side Effects::

@b{nintersection} can modify @i{list-1}, 

but not @i{list-2}.

@subsubheading  Exceptional Situations::

Should be prepared to signal an error of @i{type} @b{type-error}
			 if @i{list-1} and @i{list-2} are not @i{proper lists}.

@subsubheading  See Also::

@ref{union; nunion}
,

@ref{Compiler Terminology},

@ref{Traversal Rules and Side Effects}

@subsubheading  Notes::

The @t{:test-not} parameter is deprecated.

Since the @b{nintersection} side effect is not required,
it should not be used in for-effect-only
  positions in portable code.

@node adjoin, pushnew, intersection, Conses Dictionary
@subsection adjoin                                                           [Function]

@code{adjoin}  @i{item list {&key} key test test-not} @result{}  @i{new-list}

@subsubheading  Arguments and Values:: 

@i{item}---an @i{object}.

@i{list}---a @i{proper list}.

@i{test}---a @i{designator} for a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{test-not}---a @i{designator} for 
  a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{key}---a @i{designator} for a @i{function} of one argument,
  or @b{nil}.

@i{new-list}---a @i{list}.

@subsubheading  Description::

Tests whether @i{item} is the same as an existing element of @i{list}.
If the @i{item} is not an existing element,
@b{adjoin} adds it to @i{list} (as if by @b{cons})
and returns the resulting @i{list}; 
otherwise, nothing is added and the original @i{list} is returned.

The @i{test}, @i{test-not}, and @i{key} 
affect how it is determined whether @i{item} is the same as an @i{element} of @i{list}.
For details, see @ref{Satisfying a Two-Argument Test}.\ifvmode\else\endgraf
\ifdim \prevdepth>-1000pt 
\NIS\parskip \normalparskip\relax\fi

@subsubheading  Examples::

@example
 (setq slist '()) @result{}  NIL 
 (adjoin 'a slist) @result{}  (A) 
 slist @result{}  NIL 
 (setq slist (adjoin '(test-item 1) slist)) @result{}  ((TEST-ITEM 1)) 
 (adjoin '(test-item 1) slist) @result{}  ((TEST-ITEM 1) (TEST-ITEM 1)) 
 (adjoin '(test-item 1) slist :test 'equal) @result{}  ((TEST-ITEM 1)) 
 (adjoin '(new-test-item 1) slist :key #'cadr) @result{}  ((TEST-ITEM 1)) 
 (adjoin '(new-test-item 1) slist) @result{}  ((NEW-TEST-ITEM 1) (TEST-ITEM 1)) 
@end example

@subsubheading  Exceptional Situations::

Should be prepared to signal an error of @i{type} @b{type-error}
			 if @i{list} is not a @i{proper list}.

@subsubheading  See Also::

@ref{pushnew}
,

@ref{Traversal Rules and Side Effects}

@subsubheading  Notes::

The @t{:test-not} parameter is deprecated.

@example
 (adjoin item list :key fn)
   @equiv{} (if (member (fn item) list :key fn) list (cons item list))
@end example

@node pushnew, set-difference, adjoin, Conses Dictionary
@subsection pushnew                                                             [Macro]

@code{pushnew}  @i{item place {&key} key test test-not}@*
   @result{}  @i{new-place-value}

@subsubheading  Arguments and Values:: 

@i{item}---an @i{object}.  

@i{place}---a @i{place}, the @i{value} of which is a @i{proper list}. 

@i{test}---a @i{designator} for a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{test-not}---a @i{designator} for 
  a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{key}---a @i{designator} for a @i{function} of one argument,
  or @b{nil}.

@i{new-place-value}---a @i{list} (the new @i{value} of @i{place}).

@subsubheading  Description::

@b{pushnew} tests whether  @i{item} is the same as any existing
element of the @i{list} stored in @i{place}.  If @i{item} is not,
it is prepended to the @i{list}, and the new @i{list} is stored in
@i{place}.

@b{pushnew} returns the new @i{list} that is stored in @i{place}.

Whether or not @i{item} is already a member of the @i{list} that is
in @i{place} is determined by comparisons using @t{:test} or @t{:test-not}.
The first argument to the @t{:test} or @t{:test-not}
function is @i{item}; the second argument is
an element of the @i{list} in @i{place} as returned by
the @t{:key} function (if supplied).

If @t{:key} is supplied, it is used to extract the part to be tested from
both @i{item} and the @i{list} element,
as for @b{adjoin}.

The argument to the @t{:key} function 
is an element of the @i{list} stored in 
@i{place}. The @t{:key} function typically returns part
part of the element of the @i{list}.
If @t{:key} is not supplied or @b{nil}, the @i{list} 
element is used.

For information about the @i{evaluation} of @i{subforms} of @i{place},
see @ref{Evaluation of Subforms to Places}.

It is @i{implementation-dependent} whether or not @b{pushnew} 
actually executes the storing form for its @i{place} in the
situation where the @i{item} is already a member of the @i{list}
held by @i{place}.

@subsubheading  Examples::
@example
 (setq x '(a (b c) d)) @result{}  (A (B C) D)
 (pushnew 5 (cadr x)) @result{}  (5 B C)   
 x @result{}  (A (5 B C) D)
 (pushnew 'b (cadr x)) @result{}  (5 B C)  
 x @result{}  (A (5 B C) D)
 (setq lst '((1) (1 2) (1 2 3))) @result{}  ((1) (1 2) (1 2 3))
 (pushnew '(2) lst) @result{}  ((2) (1) (1 2) (1 2 3))
 (pushnew '(1) lst) @result{}  ((1) (2) (1) (1 2) (1 2 3))
 (pushnew '(1) lst :test 'equal) @result{}  ((1) (2) (1) (1 2) (1 2 3))
 (pushnew '(1) lst :key #'car) @result{}  ((1) (2) (1) (1 2) (1 2 3)) 
@end example

@subsubheading  Side Effects::

The contents of @i{place} may be modified.

@subsubheading  See Also::

@ref{push}
,
@ref{adjoin}
,
@ref{Generalized Reference}

@subsubheading  Notes::

The effect of
@example
 (pushnew item place :test p)
@end example

is roughly equivalent to
@example
 (setf place (adjoin item place :test p))
@end example

except that the @i{subforms} of @t{place} are evaluated only once, 
and @t{item} is evaluated before @t{place}.

@node set-difference, set-exclusive-or, pushnew, Conses Dictionary
@subsection set-difference, nset-difference                                  [Function]

@code{set-difference}  @i{list-1 list-2 {&key} key test test-not} @result{}  @i{result-list}

@code{nset-difference}  @i{list-1 list-2 {&key} key test test-not} @result{}  @i{result-list}

@subsubheading  Arguments and Values:: 

@i{list-1}---a @i{proper list}.

@i{list-2}---a @i{proper list}.

@i{test}---a @i{designator} for a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{test-not}---a @i{designator} for 
  a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{key}---a @i{designator} for a @i{function} of one argument,
  or @b{nil}.

@i{result-list}---a @i{list}.

@subsubheading  Description::
@b{set-difference} returns a @i{list} 
of elements of @i{list-1}
that do not appear in @i{list-2}.  

@b{nset-difference} is the destructive 
version of @b{set-difference}.
It may destroy @i{list-1}.

For all possible ordered pairs consisting of
one element from @i{list-1} and one element from @i{list-2}, the 
@t{:test} or @t{:test-not} function is
used to determine whether they @i{satisfy the test}.  
The first argument to the @t{:test} or @t{:test-not} function 
is the part of an element of @i{list-1} that is returned by 
the @t{:key} function (if supplied); the second argument is the part of 
an element of @i{list-2} that is 
returned by the @t{:key} function (if supplied).

If @t{:key} is supplied, its argument is a @i{list-1} or
@i{list-2} element. The @t{:key} function 
typically returns part of 
the supplied element.
If @t{:key} is not supplied, the @i{list-1} or @i{list-2}
element is used.

An element of @i{list-1}
appears in the result if and only if it does not match any element
of @i{list-2}.  

There is no guarantee that the order of elements in the result will
reflect the ordering of the arguments in any particular way.
The result @i{list} 
may share cells with, or be @b{eq} to, either of @i{list-1}
or @i{list-2},
if appropriate.

@subsubheading  Examples::

@example
 (setq lst1 (list "A" "b" "C" "d")
       lst2 (list "a" "B" "C" "d")) @result{}  ("a" "B" "C" "d")
 (set-difference lst1 lst2) @result{}  ("d" "C" "b" "A")
 (set-difference lst1 lst2 :test 'equal) @result{}  ("b" "A")
 (set-difference lst1 lst2 :test #'equalp) @result{}  NIL 
 (nset-difference lst1 lst2 :test #'string=) @result{}  ("A" "b")
 (setq lst1 '(("a" . "b") ("c" . "d") ("e" . "f")))
@result{}  (("a" . "b") ("c" . "d") ("e" . "f")) 
 (setq lst2 '(("c" . "a") ("e" . "b") ("d" . "a")))
@result{}  (("c" . "a") ("e" . "b") ("d" . "a")) 
 (nset-difference lst1 lst2 :test #'string= :key #'cdr)
@result{}  (("c" . "d") ("e" . "f")) 
 lst1 @result{}  (("a" . "b") ("c" . "d") ("e" . "f")) 
 lst2 @result{}  (("c" . "a") ("e" . "b") ("d" . "a")) 
@end example

@example
;; Remove all flavor names that contain "c" or "w".
 (set-difference '("strawberry" "chocolate" "banana"
                  "lemon" "pistachio" "rhubarb")
          '(#\c #\w)
          :test #'(lambda (s c) (find c s)))
@result{}  ("banana" "rhubarb" "lemon")    ;One possible ordering.
@end example

@subsubheading  Side Effects::

@b{nset-difference} may destroy @i{list-1}.

@subsubheading  Exceptional Situations::

Should be prepared to signal an error of @i{type} @b{type-error}
			 if @i{list-1} and @i{list-2} are not @i{proper lists}.

@subsubheading  See Also::

@ref{Compiler Terminology},

@ref{Traversal Rules and Side Effects}

@subsubheading  Notes::

The @t{:test-not} parameter is deprecated.

@node set-exclusive-or, subsetp, set-difference, Conses Dictionary
@subsection set-exclusive-or, nset-exclusive-or                              [Function]

@code{set-exclusive-or}  @i{list-1 list-2 {&key} key test test-not} @result{}  @i{result-list}

@code{nset-exclusive-or}  @i{list-1 list-2 {&key} key test test-not} @result{}  @i{result-list}

@subsubheading  Arguments and Values:: 

@i{list-1}---a @i{proper list}.

@i{list-2}---a @i{proper list}.

@i{test}---a @i{designator} for a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{test-not}---a @i{designator} for 
  a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{key}---a @i{designator} for a @i{function} of one argument,
  or @b{nil}.

@i{result-list}---a @i{list}.

@subsubheading  Description::
@b{set-exclusive-or} returns a @i{list} of elements that appear
in exactly one of @i{list-1} and @i{list-2}.

@b{nset-exclusive-or} 
is the @i{destructive} version of @b{set-exclusive-or}.

For all possible ordered pairs consisting of
one element from @i{list-1} and one element from @i{list-2}, the 
@t{:test} or @t{:test-not} function is
used to determine whether they @i{satisfy the test}.  

If @t{:key} is supplied, it is used to
extract the part to be tested from the @i{list-1} or @i{list-2} element. 
The first argument to the @t{:test} or @t{:test-not} function 
is the part of an element of @i{list-1} extracted by the @t{:key}
function (if supplied); the second argument  is the part of an 
element of @i{list-2} extracted by the @t{:key} function (if supplied).
If @t{:key} is not supplied or @b{nil}, the @i{list-1} or
@i{list-2} element is used.

The result contains precisely
those elements of @i{list-1} and @i{list-2} 
that appear in no matching pair.

The result @i{list} of @b{set-exclusive-or} 
might share storage with one of @i{list-1} or @i{list-2}.

@subsubheading  Examples::

@example
 (setq lst1 (list 1 "a" "b")
       lst2 (list 1 "A" "b")) @result{}  (1 "A" "b")
 (set-exclusive-or lst1 lst2) @result{}  ("b" "A" "b" "a")
 (set-exclusive-or lst1 lst2 :test #'equal) @result{}  ("A" "a")
 (set-exclusive-or lst1 lst2 :test 'equalp) @result{}  NIL 
 (nset-exclusive-or lst1 lst2) @result{}  ("a" "b" "A" "b") 
 (setq lst1 (list (("a" . "b") ("c" . "d") ("e" . "f"))))
@result{}  (("a" . "b") ("c" . "d") ("e" . "f"))
 (setq lst2 (list (("c" . "a") ("e" . "b") ("d" . "a"))))
@result{}  (("c" . "a") ("e" . "b") ("d" . "a")) 
 (nset-exclusive-or lst1 lst2 :test #'string= :key #'cdr)
@result{}  (("c" . "d") ("e" . "f") ("c" . "a") ("d" . "a")) 
 lst1 @result{}  (("a" . "b") ("c" . "d") ("e" . "f"))
 lst2 @result{}  (("c" . "a") ("d" . "a")) 
@end example

@subsubheading  Side Effects::

@b{nset-exclusive-or} is permitted to modify any part,
@i{car} or @i{cdr}, of the @i{list structure} of @i{list-1} or @i{list-2}.

@subsubheading  Exceptional Situations::

Should be prepared to signal an error of @i{type} @b{type-error}
			 if @i{list-1} and @i{list-2} are not @i{proper lists}.

@subsubheading  See Also::

@ref{Compiler Terminology},

@ref{Traversal Rules and Side Effects}

@subsubheading  Notes::

The @t{:test-not} parameter is deprecated.

Since the @b{nset-exclusive-or} side effect is not required,
it should not be used in for-effect-only
  positions in portable code.

@node subsetp, union, set-exclusive-or, Conses Dictionary
@subsection subsetp                                                          [Function]

@code{subsetp}  @i{list-1 list-2 {&key} key test test-not} @result{}  @i{generalized-boolean}

@subsubheading  Arguments and Values::

@i{list-1}---a @i{proper list}.

@i{list-2}---a @i{proper list}.

@i{test}---a @i{designator} for a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{test-not}---a @i{designator} for 
  a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{key}---a @i{designator} for a @i{function} of one argument,
  or @b{nil}.

@i{generalized-boolean}---a @i{generalized boolean}.

@subsubheading  Description::

@b{subsetp} returns @i{true} if every element of @i{list-1}
matches some element of @i{list-2},
and @i{false} otherwise.

Whether a list element is the same as another list element is
determined by the functions specified by the keyword arguments.  
The first argument to the @t{:test} or @t{:test-not} 
function is 
typically
part of an element of @i{list-1} extracted by
the @t{:key} function; the second argument is  typically part of 
an element of @i{list-2} extracted by
the @t{:key} function.

The argument to the @t{:key} function is an element of either
@i{list-1} or @i{list-2}; the return value is part of the element
of the supplied list element.
If @t{:key} is not supplied or @b{nil}, 
the @i{list-1} or @i{list-2}
element itself is supplied to the @t{:test} or @t{:test-not} 
function.

@subsubheading  Examples::

@example
 (setq cosmos '(1 "a" (1 2))) @result{}  (1 "a" (1 2))
 (subsetp '(1) cosmos) @result{}  @i{true}
 (subsetp '((1 2)) cosmos) @result{}  @i{false}
 (subsetp '((1 2)) cosmos :test 'equal) @result{}  @i{true}
 (subsetp '(1 "A") cosmos :test #'equalp) @result{}  @i{true}
 (subsetp '((1) (2)) '((1) (2))) @result{}  @i{false}
 (subsetp '((1) (2)) '((1) (2)) :key #'car) @result{}  @i{true}
@end example

@subsubheading  Exceptional Situations::

Should be prepared to signal an error of @i{type} @b{type-error}
			 if @i{list-1} and @i{list-2} are not @i{proper lists}.

@subsubheading  See Also::

@ref{Traversal Rules and Side Effects}

@subsubheading  Notes::

The @t{:test-not} parameter is deprecated.

@node union,  , subsetp, Conses Dictionary
@subsection union, nunion                                                    [Function]

@code{union}  @i{list-1 list-2 {&key} key test test-not} @result{}  @i{result-list}

@code{nunion}  @i{list-1 list-2 {&key} key test test-not} @result{}  @i{result-list}

@subsubheading  Arguments and Values:: 

@i{list-1}---a @i{proper list}.

@i{list-2}---a @i{proper list}.

@i{test}---a @i{designator} for a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{test-not}---a @i{designator} for 
  a @i{function} of two @i{arguments}
  that returns a @i{generalized boolean}.

@i{key}---a @i{designator} for a @i{function} of one argument,
  or @b{nil}.

@i{result-list}---a @i{list}.

@subsubheading  Description::

@b{union} and @b{nunion} return a @i{list}
that contains every element that occurs in either @i{list-1} 
or @i{list-2}.                                 

For all possible ordered pairs consisting of one
element from @i{list-1} 
and one element from @i{list-2}, @t{:test} or  @t{:test-not} is used
to determine whether they @i{satisfy the test}. 
The first argument to the @t{:test} or @t{:test-not} 
function is the part of the element of @i{list-1} extracted by the
@t{:key} function (if supplied); the second argument  
is the part of the element of @i{list-2} extracted by the
@t{:key} function (if supplied).

The argument to the @t{:key} function is an element of
@i{list-1} or @i{list-2}; the return value is part of the supplied
element.  
If @t{:key} is not supplied or @b{nil}, 
the element of @i{list-1} or @i{list-2}
itself is supplied to the @t{:test} or @t{:test-not} function. 

For every matching pair, 
one of the two elements of the pair will be in the result.  Any
element from either @i{list-1} or @i{list-2}
that matches no element of the other will appear
in the result.  

If there is a duplication between @i{list-1} 
and @i{list-2},
only one of the duplicate instances will be in the result.
If either @i{list-1} 
or @i{list-2} has duplicate entries within it,
the redundant entries
might or might not appear in the result.

The order of elements in the result do not have to
reflect the ordering of @i{list-1} or @i{list-2} in any way.
The result @i{list} may be @b{eq} to either
@i{list-1} or @i{list-2} if appropriate.

@subsubheading  Examples::

@example
 (union '(a b c) '(f a d))
@result{}  (A B C F D)
@i{OR}@result{} (B C F A D)
@i{OR}@result{} (D F A B C)
 (union '((x 5) (y 6)) '((z 2) (x 4)) :key #'car)
@result{}  ((X 5) (Y 6) (Z 2))
@i{OR}@result{} ((X 4) (Y 6) (Z 2))

 (setq lst1 (list 1 2 '(1 2) "a" "b")
       lst2 (list 2 3 '(2 3) "B" "C"))
@result{}  (2 3 (2 3) "B" "C")
 (nunion lst1 lst2)
@result{}  (1 (1 2) "a" "b" 2 3 (2 3) "B" "C") 
@i{OR}@result{} (1 2 (1 2) "a" "b" "C" "B" (2 3) 3)
@end example

@subsubheading  Side Effects::

@b{nunion} is permitted to modify any part, @i{car} or @i{cdr}, 
of the @i{list structure} of @i{list-1} or @i{list-2}.

@subsubheading  Exceptional Situations::

Should be prepared to signal an error of @i{type} @b{type-error}
			 if @i{list-1} and @i{list-2} are not @i{proper lists}.

@subsubheading  See Also::

@ref{intersection; nintersection}
,

@ref{Compiler Terminology},

@ref{Traversal Rules and Side Effects}

@subsubheading  Notes::

The @t{:test-not} parameter is deprecated.

Since the @b{nunion} side effect is not required,
it should not be used in for-effect-only positions in portable code.

@c end of including dict-conses

@c %**end of chapter