(defun path-length (path) (if (null (rest path)) 0 (+ (node-distance (first path) (second path)) (path-length (rest path)))))ふたつのパスを比較する際に, 現在までに辿ってきた道のりのコストの総和がどちらが小さいかを 調べる関数shorterpは以下のようになります.
(defun shorterp (path-1 path-2) (< (path-length path-1) (path-length path-2)))