add functionality to delete pixel

fcore/normal-mode.sml

@@ -180,6 +180,47 @@ struct
     in addCoordinates (model, arrowX, arrowY)
     end
 
+  fun deletePixel (model, hIdx, vIdx) =
+    let
+      val
+        { windowWidth
+        , windowHeight
+        , xClickPoints
+        , yClickPoints
+        , canvasWidth
+        , canvasHeight
+        , ...
+        } = model
+
+      val xpos = Vector.sub (xClickPoints, hIdx)
+      val ypos = Vector.sub (yClickPoints, vIdx)
+
+      val model = AppWith.deleteSquare (model, hIdx, vIdx, hIdx, vIdx)
+      val squares = #squares model
+
+      val dotVec = getDotVecFromIndices (model, hIdx, vIdx)
+
+      val maxSide = Int.max (canvasWidth, canvasHeight)
+      val squares = CollisionTree.toTriangles
+        ( windowWidth
+        , windowHeight
+        , squares
+        , maxSide
+        , canvasWidth
+        , canvasHeight
+        , xClickPoints
+        , yClickPoints
+        )
+      val drawMsg = DRAW_SQUARES_AND_DOTS {squares = squares, dots = dotVec}
+    in
+      (model, [DRAW drawMsg])
+    end
+
+  fun backspace model =
+    let val {arrowX, arrowY, ...} = model
+    in deletePixel (model, arrowX, arrowY)
+    end
+
   fun resizeWindow (model, width, height) =
     let
       val model = AppWith.windowResize (model, width, height)
@@ -355,6 +396,7 @@ struct
     | ARROW_LEFT => moveArrowLeft model
     | ARROW_RIGHT => moveArrowRight model
     | ARROW_DOWN => moveArrowDown model
+    | KEY_BACKSPACE => backspace model
     | KEY_ENTER => enterOrSpaceCoordinates model
     | KEY_SPACE => enterOrSpaceCoordinates model
     | FILE_BROWSER_AND_PATH {fileBrowser, path} =>