Undo and Redo
Two options in the edit menu show the possibility to reverse and re-reverse the last executed add/insert, copy, paste or edit command. These can also be performed by using the well-known keyboard shortcuts Ctrl-Z and Ctrl-Y. The options in the menu will be ungreyed whenever any of these actions becomes available.
In the File menu there's also an option that provides support for an unlimited number of undo/redo actions. Simply choose the menu option and this support is activated, recognizable by a checkmark appearing on the menu option. In the same way you can also disable this support, and the checkmark will disappear accordingly.
This option is somewhat hidden (i.e. not reachable with a keyboard shortcut) because it has a serious drawback: sometimes the undo/redo stack loses the context of an action and redoes it the wrong way or even not at all. So caution on this is advised.