reimplement parsing code (but program does not compile yet because I have to change type of 'USE_SQUARES' constructure)

fcore/layer-tree.sml

@@ -9,6 +9,9 @@ struct
     in NODE {key = minKey, value = grid, left = LEAF, right = LEAF}
     end
 
+  fun singleton grid =
+    NODE {key = minKey, value = grid, left = LEAF, right = LEAF}
+
   fun insert (newKey, newValue, tree) =
     case tree of
       LEAF => NODE {key = newKey, value = newValue, left = LEAF, right = LEAF}