add functionality to delete pixel

fcore/app-with.sml

@@ -56,11 +56,6 @@ struct
         , redo
         } = app
 
-      val yAxis = Vector.sub (squares, newX)
-      val undoData = Vector.sub (yAxis, newY)
-      val undo = undoData :: undo
-      val redo = []
-
       val item = {r = r, g = g, b = b, a = a}
       val squares = updateSquares (squares, newX, newY, item)
     in
@@ -91,6 +86,65 @@ struct
       }
     end
 
+  fun deleteSquare (app, deleteX, deleteY, arrowX, arrowY) =
+    let
+      val
+        { mode
+        , squares
+        , arrowX = _
+        , arrowY = _
+        , canvasWidth
+        , canvasHeight
+        , windowWidth
+        , windowHeight
+        , xClickPoints
+        , yClickPoints
+
+        , showGraph
+        , mouseX
+        , mouseY
+        , openFilePath
+        , fileBrowser
+        , fileBrowserIdx
+        , r
+        , g
+        , b
+        , a
+        , modalNum
+        , undo
+        , redo
+        } = app
+
+      val item = {r = 0, g = 0, b = 0, a = 0}
+      val squares = updateSquares (squares, deleteX, deleteY, item)
+    in
+      { mode = mode
+      , squares = squares
+      , arrowX = arrowX
+      , arrowY = arrowY
+      , canvasWidth = canvasWidth
+      , canvasHeight = canvasHeight
+      , windowWidth = windowWidth
+      , windowHeight = windowHeight
+      , xClickPoints = xClickPoints
+      , yClickPoints = yClickPoints
+
+      , showGraph = showGraph
+      , mouseX = mouseX
+      , mouseY = mouseY
+      , openFilePath = openFilePath
+      , fileBrowser = fileBrowser
+      , fileBrowserIdx = fileBrowserIdx
+      , r = r
+      , g = g
+      , b = b
+      , a = a
+      , modalNum = modalNum
+      , undo = undo
+      , redo = redo
+      }
+    end
+
   fun arrowX (app, arrowX) =
     let
       val