change BNF (but not implementation of BNF) for parsing, to handle layers, and also change minimum key in layer tree to be 1

fcore/layer-tree.sml

@@ -2,9 +2,11 @@ structure LayerTree =
 struct
   datatype t = NODE of {key: int, value: Grid.t, left: t, right: t} | LEAF
 
+  val minKey = 1
+
   fun init maxSide =
     let val grid = Grid.makeEmpty maxSide
-    in NODE {key = 0, value = grid, left = LEAF, right = LEAF}
+    in NODE {key = minKey, value = grid, left = LEAF, right = LEAF}
     end
 
   fun insert (newKey, newValue, tree) =