progress moving away from 'squares' field to layer tree

fcore/layer-tree.sml

@@ -2,7 +2,10 @@ structure LayerTree =
 struct
   datatype t = NODE of {key: int, value: Grid.t, left: t, right: t} | LEAF
 
-  val empty = LEAF
+  fun init maxSide =
+    let val grid = Grid.makeEmpty maxSide
+    in NODE {key = 0, value = grid, left = LEAF, right = LEAF}
+    end
 
   fun insert (newKey, newValue, tree) =
     case tree of
@@ -82,7 +85,4 @@ struct
     in
       insert (key, grid, tree)
     end
-
-  fun removePixel (key, newX, newY, maxSide, tree) =
-    addPixel (key, newX, newY, maxSide, Grid.emptyPixel, tree)
 end