Robot Mouse in Maze

Depth-first search of an implicit tree can simulate a robot mouse in a maze. The goal is to return a sequence of steps to guide the mouse to the cheese.

(defn mouse [maze x y prev]
(if (or (= (nth (nth maze y) x) '*) ; hit wall
        (member (list x y) prev)) ; been there
    nil                           ; fail
    (if (= (nth (nth maze y) x) 'c) ; cheese
        '()               ; success
        (let [path (mouse maze (- x 1) y ; go west
                          (cons (list x y) prev))]
          (if path (cons 'w path)
              (let [path (mouse maze x  (- y 1)
                          (cons (list x y) prev))]
               (if path (cons 'n path)
                   (let [path (mouse maze (+ x 1) y
                          (cons (list x y) prev))]
                    (if path (cons 'e path)
                        (let [path (mouse maze x (+ y 1)
                          (cons (list x y) prev))]
                          (if path (cons 's path)
                              nil)))))))))))     ; fail