diff --git a/scripts/Common.gd b/scripts/Common.gd
index ac097b5..884ba98 100644
--- a/scripts/Common.gd
+++ b/scripts/Common.gd
@@ -58,69 +58,3 @@ class IntArray2D extends PoolArray2D:
 
 	func clear_flag(row: int, col: int, value: int):
 		_array.set(row + col*_rows, _array[row + col*_rows]&(value^XORMASK))
-
-
-#class ByteArray2D:
-#	var _array: PoolByteArray
-#	var _rows := 1
-#	var _cols := 1
-#	const XORMASK := (1<<8)-1
-#	func _init(rows:=1, cols:=1):
-#		assert(rows>0)
-#		assert(cols>0)
-#		_rows = rows
-#		_cols = cols
-#		_array = PoolByteArray()
-#		_array.resize(rows*cols)
-#
-#	func set_cell(row: int, col: int, value: int):
-#		assert(row < _rows)
-#		assert(col < _cols)
-#		_array.set(row + col*_rows, value)
-#
-#	func get_cell(row: int, col: int) -> int:
-#		assert(row < _rows)
-#		assert(col < _cols)
-#		return _array[row + col*_rows]
-#
-#	func get_flag(row: int, col: int, value: int) -> bool:
-#		return bool(_array[row + col*_rows] & value)
-#
-#	func set_flag(row: int, col: int, value: int):
-#		_array.set(row + col*_rows, _array[row + col*_rows]|value)
-#
-#	func clear_flag(row: int, col: int, value: int):
-#		_array.set(row + col*_rows, _array[row + col*_rows]&(value^XORMASK))
-
-
-#class IntArray2D:
-#	var _array: PoolIntArray
-#	var _rows := 1
-#	var _cols := 1
-#	const XORMASK := (1<<32)-1
-#	func _init(rows:=1, cols:=1):
-#		assert(rows>0)
-#		assert(cols>0)
-#		_rows = rows
-#		_cols = cols
-#		_array = PoolIntArray()
-#		_array.resize(rows*cols)
-#
-#	func set_cell(row: int, col: int, value: int):
-#		assert(row < _rows)
-#		assert(col < _cols)
-#		_array.set(row + col*_rows, value)
-#
-#	func get_cell(row: int, col: int) -> int:
-#		assert(row < _rows)
-#		assert(col < _cols)
-#		return _array[row + col*_rows]
-#
-#	func get_flag(row: int, col: int, value: int) -> bool:
-#		return bool(_array[row + col*_rows] & value)
-#
-#	func set_flag(row: int, col: int, value: int):
-#		_array.set(row + col*_rows, _array[row + col*_rows] | value)
-#
-#	func clear_flag(row: int, col: int, value: int):
-#		_array.set(row + col*_rows, _array[row + col*_rows]&(value^XORMASK))
diff --git a/scripts/GameField.gd b/scripts/GameField.gd
index d709f20..283d46b 100644
--- a/scripts/GameField.gd
+++ b/scripts/GameField.gd
@@ -242,26 +242,60 @@ func _input(event: InputEvent) -> void:
 			match drag_action:
 				DragAction.DRAW_LINE:
 					if adx < threshold and ady > threshold:
-						corner_marks.set_flag(int(gridpos.y), int(round(gridpos.x)), CornerMark.L_DOWN)
+						set_line_down(int(gridpos.y), int(round(gridpos.x)))
 					elif adx > threshold and ady < threshold:
-						corner_marks.set_flag(int(round(gridpos.y)), int(gridpos.x), CornerMark.L_RIGHT)
+						set_line_right(int(round(gridpos.y)), int(gridpos.x))
 				DragAction.REMOVE_LINE:
 					if adx < threshold and ady > threshold:
-						corner_marks.clear_flag(int(gridpos.y), int(round(gridpos.x)), CornerMark.L_DOWN)
+						clear_line_down(int(gridpos.y), int(round(gridpos.x)))
 					elif adx > threshold and ady < threshold:
-						corner_marks.clear_flag(int(round(gridpos.y)), int(gridpos.x), CornerMark.L_RIGHT)
+						clear_line_right(int(round(gridpos.y)), int(gridpos.x))
 				DragAction.DRAW_X:
 					if adx < threshold and ady > threshold:
-						corner_marks.set_flag(int(gridpos.y), int(round(gridpos.x)), CornerMark.X_DOWN)
+						set_x_down(int(gridpos.y), int(round(gridpos.x)))
 					elif adx > threshold and ady < threshold:
-						corner_marks.set_flag(int(round(gridpos.y)), int(gridpos.x), CornerMark.X_RIGHT)
+						set_x_right(int(round(gridpos.y)), int(gridpos.x))
 				DragAction.REMOVE_X:
 					if adx < threshold and ady > threshold:
-						corner_marks.clear_flag(int(gridpos.y), int(round(gridpos.x)), CornerMark.X_DOWN)
+						clear_x_down(int(gridpos.y), int(round(gridpos.x)))
 					elif adx > threshold and ady < threshold:
-						corner_marks.clear_flag(int(round(gridpos.y)), int(gridpos.x), CornerMark.X_RIGHT)
+						clear_x_right(int(round(gridpos.y)), int(gridpos.x))
 				DragAction.DRAW_COLOR:
 					sel_row = int(gridpos.y)
 					sel_col = int(gridpos.x)
 					cell_colors.set_cell(sel_row, sel_col, drag_color)
 
+var undo_stack = []
+
+func set_line_right(row, col):
+	if !corner_marks.get_flag(row, col, CornerMark.L_RIGHT) and !corner_marks.get_flag(row, col, CornerMark.X_RIGHT):
+		corner_marks.set_flag(row, col, CornerMark.L_RIGHT)
+		# TODO: Add undo stack entry
+
+func set_line_down(row, col):
+	if !corner_marks.get_flag(row, col, CornerMark.L_DOWN) and !corner_marks.get_flag(row, col, CornerMark.X_DOWN):
+		corner_marks.set_flag(row, col, CornerMark.L_DOWN)
+
+func clear_line_right(row, col):
+	if corner_marks.get_flag(row, col, CornerMark.L_RIGHT):
+		corner_marks.clear_flag(row, col, CornerMark.L_RIGHT)
+
+func clear_line_down(row, col):
+	if corner_marks.get_flag(row, col, CornerMark.L_DOWN):
+		corner_marks.clear_flag(row, col, CornerMark.L_DOWN)
+
+func set_x_right(row, col):
+	if !corner_marks.get_flag(row, col, CornerMark.L_RIGHT) and !corner_marks.get_flag(row, col, CornerMark.X_RIGHT):
+		corner_marks.set_flag(row, col, CornerMark.X_RIGHT)
+
+func set_x_down(row, col):
+	if !corner_marks.get_flag(row, col, CornerMark.L_DOWN) and !corner_marks.get_flag(row, col, CornerMark.X_DOWN):
+		corner_marks.set_flag(row, col, CornerMark.X_DOWN)
+
+func clear_x_right(row, col):
+	if corner_marks.get_flag(row, col, CornerMark.X_RIGHT):
+		corner_marks.clear_flag(row, col, CornerMark.X_RIGHT)
+
+func clear_x_down(row, col):
+	if corner_marks.get_flag(row, col, CornerMark.X_DOWN):
+		corner_marks.clear_flag(row, col, CornerMark.X_DOWN)