Undo and Redo operations are live!

Here is something super important for a math puzzle game: undo and redo!  You can now use the arrow buttons at the top left corner of the screen to undo or redo operations.  You can also press Ctrl + Z to undo and Ctrl + Shift + Z to redo from the keyboard.  There will be a catch though: each time you undo and redo, it'll count against your high score.  Try not to make a mistake if you're shooting for the least possible moves!