diff --git a/fcore/quad-tree-fold.sml b/fcore/quad-tree-fold.sml
index f0ca4b0..afc9133 100644
--- a/fcore/quad-tree-fold.sml
+++ b/fcore/quad-tree-fold.sml
@@ -9,12 +9,50 @@ functor MakeQuadFolder(Fn: QUAD_FOLDER) =
 struct
   open QuadTreeType
 
+  fun visitTopLeft (iX, iY, iW, iH, qX, qY, qW, qH) =
+    let
+      val midX = qW div 2 + qX
+      val midY = qH div 2 + qY
+    in
+      iX <= midX andalso iY <= midY
+    end
+
+  fun visitTopRight (iX, iY, iW, iH, qX, qY, qW, qH) =
+    let
+      val midX = qW div 2 + qX
+      val midY = qH div 2 + qY
+    in
+      iX >= midX andalso iY <= midY
+    end
+
+  fun visitBottomLeft (iX, iY, iW, iH, qX, qY, qW, qH) =
+    let
+      val midX = qW div 2 + qX
+      val midY = qH div 2 + qY
+
+      val iFinishY = iY + iH
+    in
+      iX <= midX andalso iFinishY >= midY
+    end
+
+  fun visitBottomRight (iX, iY, iW, iH, qX, qY, qW, qH) =
+    let
+      val midX = qW div 2 + qX
+      val midY = qH div 2 + qY
+
+      val iFinishX = iX + iH
+      val iFinishY = iY + iH
+    in
+      iFinishX >= midX andalso iFinishY >= midY
+    end
+
   fun foldVec (iX, iY, iW, iH, pos, elements, state, env) =
     if pos = Vector.length elements then
       state
     else
       let
         val {itemID, ...} = Vector.sub (elements, pos)
+        val _ = print ("foldVec itemID: " ^ Int.toString itemID ^ "\n")
         val state = Fn.fState (state, env, itemID)
       in
         foldVec (iX, iY, iW, iH, pos + 1, elements, state, env)
@@ -42,68 +80,84 @@ struct
 
           val halfW = quadW div 2
           val halfH = quadH div 2
+
+          val midX = halfW + quadX
+          val midY = halfH + quadY
+
+          val iX = itemX
+          val iY = itemY
+          val iW = itemW
+          val iH = itemH
+
+          val qX = quadX
+          val qY = quadY
+          val qW = quadW
+          val qH = quadH
+
+          val vtl = visitTopLeft (iX, iY, iW, iH, qX, qY, qW, qH)
+          val vtr = visitTopRight (iX, iY, iW, iH, qX, qY, qW, qH)
+          val vbl = visitBottomLeft (iX, iY, iW, iH, qX, qY, qW, qH)
+          val vbr = visitBottomRight (iX, iY, iW, iH, qX, qY, qW, qH)
+
+          val state =
+            if vtl then
+              foldRegion
+                (iX, iY, iW, iH, qX, qY, halfW, halfH, env, state, topLeft)
+            else
+              state
+
+          val state =
+            if vtr then
+              foldRegion
+                ( itemX
+                , itemY
+                , itemW
+                , itemH
+                , midX
+                , quadY
+                , halfW
+                , halfH
+                , env
+                , state
+                , topRight
+                )
+            else
+              state
+
+          val state =
+            if vbl then
+              foldRegion
+                ( itemX
+                , itemY
+                , itemW
+                , itemH
+                , quadX
+                , midY
+                , halfW
+                , halfH
+                , env
+                , state
+                , bottomLeft
+                )
+            else
+              state
         in
-          (case
-             QuadTree.whichQuadrant
-               (itemX, itemY, itemW, itemH, quadX, quadY, quadW, quadH)
-           of
-             TOP_LEFT =>
-               foldRegion
-                 ( itemX
-                 , itemY
-                 , itemW
-                 , itemH
-                 , quadX
-                 , quadY
-                 , halfW
-                 , halfH
-                 , env
-                 , state
-                 , topLeft
-                 )
-           | TOP_RIGHT =>
-               foldRegion
-                 ( itemX
-                 , itemY
-                 , itemW
-                 , itemH
-                 , quadX + halfW (* middleX *)
-                 , quadY
-                 , halfW
-                 , halfH
-                 , env
-                 , state
-                 , topRight
-                 )
-           | BOTTOM_LEFT =>
-               foldRegion
-                 ( itemX
-                 , itemY
-                 , itemW
-                 , itemH
-                 , quadX
-                 , quadY + halfH (* middleY *)
-                 , halfW
-                 , halfH
-                 , env
-                 , state
-                 , bottomLeft
-                 )
-           | BOTTOM_RIGHT =>
-               foldRegion
-                 ( itemX
-                 , itemY
-                 , itemW
-                 , itemH
-                 , quadX + halfW (* middleX *)
-                 , quadY + halfH (* middleY *)
-                 , halfW
-                 , halfH
-                 , env
-                 , state
-                 , bottomRight
-                 )
-           | PARENT_QUADRANT => state)
+          if vbr then
+            foldRegion
+              ( itemX
+              , itemY
+              , itemW
+              , itemH
+              , midX
+              , midY
+              , halfW
+              , halfH
+              , env
+              , state
+              , bottomRight
+              )
+          else
+            state
         end
     | LEAF elements =>
         foldVec (itemX, itemY, itemW, itemH, 0, elements, state, env)