These are my notes for how to implement an undo for the format command.
I think the strategy is that I have a special FORMAT type for the undo. If the UNDO.HISTORY or REDO.HISTORY subroutines hit that type, they load in the saved raw file at that point in time. This feels like it would be a pretty big memory hog given that I would basically save a copy of the entire file anytime a user calls the format command.
I think this would be simplest way to implement it but I'm not a big fan of it. However simplicity beats out everything else and I really do need a format undo. I currently just exit and reopen the file but this would be quite the lifesaver. Especially since I would get a much longer history now though it would be quite heavy.
I would probably not save the formatting so that may be an issue.