routes.clj

(defn routes
  "Calculates all the routes from [x y] to [0 0].
  Routes move only down and left."
  [x y]
  ((fn routes* [queue]
     (lazy-seq
      (loop [queue queue]
	(when (pos? (count queue))
	  (let [[current-level [x y]] (peek queue)
		queue-left            (pop queue)
		result-level (conj current-level [x y])]
	    (if (= 0 x y)
	      (cons result-level (routes* queue-left))
	      (recur 
	       (into queue-left
		     (for [new-pos [(when (pos? x) [(dec x) y])
				    (when (pos? y) [x (dec y)])]
			   :when new-pos]
		       [result-level new-pos])))))))))
   (conj clojure.lang.PersistentQueue/EMPTY [[] [x y]])))