consecutive-fibonacci-numbers-are-coprime: Refactor gcd$ function.

1 files changed, 7 insertions, 7 deletions

diff --git a/consecutive-fibonacci-numbers-are-coprime.lisp b/consecutive-fibonacci-numbers-are-coprime.lisp
index e9bc9e2..1700e8e 100644
--- a/consecutive-fibonacci-numbers-are-coprime.lisp
+++ b/consecutive-fibonacci-numbers-are-coprime.lisp
@@ -17,13 +17,13 @@
   (declare (xargs :measure (+ (nfix m) (nfix n))
                   :guard (and (posp n)
                               (posp m))))
-  (cond
-    ((or (not (posp n))
-         (not (posp m))
-         (= (nfix n) (nfix m)))
-     n)
-    ((< n m) (gcd$ (- m n) n))
-    (t (gcd$ (- n m) m))))
+  (if (and (posp n) (posp m))
+      (cond
+        ((= (nfix n) (nfix m))
+         n)
+        ((< n m) (gcd$ (- m n) n))
+        (t (gcd$ (- n m) m)))
+      0))
 
 ;; gcd$ の結果は正の整数である
 (defthm posp-gcd$