Allow zero coordinates in cells (d'oh!)

src/cljc/mw_engine/world.clj

@@ -38,19 +38,20 @@
 (defn cell?
   "Return `true` if `obj` is a cell, as understood by MicroWorld, else `false`."
   [obj]
+  (let [x (:x obj)
+        y (:y obj)]
    (and (map? obj) ;; it's a map...
        ;; TODO: it's worth checking (and this does not) that cells have the
        ;; right co-ordinates!
-       (pos-int? (:x obj)) ;; with an x co-ordinate...
-       (pos-int? (:y obj)) ;; and a y co-ordinate...
-       (keyword? (:state obj)))) ;; and a state which is a keyword.
+       (or (zero? x)(pos-int? x)) ;; with an x co-ordinate...
+       (or (zero? y)(pos-int? y)) ;; and a y co-ordinate...
+       (keyword? (:state obj))))) ;; and a state which is a keyword.
 
 (defn world?
   "Return `true` if `obj` is a world, as understood by MicroWorld, else `false`."
   [obj]
   (and (coll? obj) ;; it's a collection...
        (every? coll? obj) ;; of collections...
-       (= 1 (count (set (map count obj)))) ;; all of which are the same length...
        (every? cell? (flatten obj)))) ;; and every element of each of those is a cell.
 
 (defmacro make-cell