Er! I say "er" when I've said a word and it was incorrect. That's not really saying enough about it: I don't think it's really possible to mis-say. Speaking is part of thinking. Speaking is a mind-organ I have which performs the function of linearising thoughts. So I'm not mis-saying when I say, um, words, er, speak, er, express some concept but don't say it right the first time round. I'm iterating. I'm making a depth-first search of possible linear packs and I hit a dead-end. I'm thinking by speaking. The "er" there wasn't just a busy cursor. It was a verbal backspace keypress.
The "er" isn't just an inward facing signal. I say it to step backwards and ask all listeners to disregard my previous word or two. I suspect this signal is so strong that you could do trials, er, experiments, and listeners would actually here the erred word less. It would be backwards masked.
From this perspective, how could Undo on my computer be different? "Undo" is functional "er" in a way. To say "er" is to safely experiment with a way forward. If my computer had pervasive "er", I'd try out different routes forwards more playfully. Say, in Photoshop, I'd try one sequence of actions, then if I was happy I'd commit to it and if I wasn't I'd, er, try another. How is this different to undoing now? I think the very name "undo" implies I've made some kind of mistake or error. No! Doing is part of thinking. To say that I can make a mistake when writing or photoshopping is saying I'm like a computer operator, dominated by time-and-motion and effectiveness. I'm not a machine. Undo makes sense if I slip with the mouse and choose the wrong menu item; its use for trying things out is an unintended consequence.
My computer needs a speculative mode. When I'm in Excel, I should be able to drop into a sandbox where I can play with my spreadsheet. Maybe reorganising this, calculating that, seeing what happens if I change these fonts. In the sandbox, "Save" should be switched off. There should be no way for the document to save. If I'm happy with it, those actions (maybe just a couple of them) should be re-run on the real document. If I'm not happy, no harm done. How often do I try things out, figure out it doesn't work, and end up having to painfully backtrack by making a half dozen small revisions? The "Revert" function is not good enough when you're working on 20 interlinked files. Version control is too heavy-weight when you're playing like this.
My computer needs a new function, pervasive er. It should be promoted to its own key on the keyboard. I'll steal Jack's joke: We could call it Umdo.