Several posts have suggested this, so I will add my vote.
There should be a Redo feature. All too often, I press Ctrl-Z thinking that it will undo a typing change I've just made to a single field-cell. Of course, Ctrl-Z actually undoes the last "big" change you've made using Tag-Tag or a similar feature. Unfortunately, when you Undo something like that: (1) you can't redo it with a keyboard shortcut or menu item (as these don't exist) and (2) you often can't remember what exactly your last change was so that you can go back and redo it yourself.
Relatedly, it would help to have a feature to save the tag-list in its current state with no availability of Undo for any past changes (equivalent to shutting down the program and restarting it).
The Mac keyboard shortcut list appears to reflect that Redo is available to Mac users -- ??