I got an idea add a undo button

[Not my name]

This could be useful since their is many operations you can do on OS, that can cause changes that are extremely annoying to remove. If you make a mistake when pasting in a section, it can ruin an entire sequence for example.

[everypony]

undo:
Windows/Linux: Ctrl Z
Mac: ⌘ Z

redo:
Windows/Linux: Ctrl Shift Z
Mac: ⌘ ⇧ Z
(or)
Windows/Linux: Ctrl Y
Mac: ⌘ Y