Update euclid to return nil on negative args

chapter-1/euclid.lisp

@@ -13,15 +13,20 @@
 
 (defun euclid (m n)
   "Calculates the common divisor between m and n"
-  (unless (> m n)
-    (euclid n m))
-  (let ((r (mod m n)))
-    (if (= r 0)
-        n
-        (euclid n r))))
+  (when (> 0 m n)
+    (return-from euclid nil))
+  (if (< m n)
+      (euclid n m)
+      (let ((r (mod m n)))
+        (if (= r 0)
+            n
+            (euclid n r)))))
 
 ;; Tests
 (ql:quickload "prove")
 (prove::plan 3)
 (prove::is (euclid 235 95) 5)
 (prove::is (euclid 119 544) 17)
+(prove::is (euclid -5 94) nil)
+(prove::is (euclid 94 -5) nil)
+(prove::is (euclid -94 -5) nil)