(deftype game-map [xdim ydim contents]
  clojure.lang.IPersistentVector
  (assoc [this key val]
    (game-map. xdim ydim
              (assoc contents (+ (first key) (* (second key) ydim)) val)))
  clojure.lang.IFn
  (invoke [this col]
          (get contents (+ (first col) (* (second col) ydim)))))