Reading the comments on how vim and Emacs have implemented Undo/Redo with various options was very helpful. I personally don't use these editors, but I'm working on a graph drawing tool where the user usually modifies multiple parts of the graph. Right now we only have linear redo/undo implemented. Reading about all this other options is really helpful. Especially the Time travel and the regional undo are very smart features that are also very helpful for all graphic based editors.