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Command-based interfaces can use both control keys and text commands. In the case of "undo", it will indeed require less key presses to use the control key combination than to bring up the HUD, then type "undo".
But what about the song selection example? A limitation of control keys in command interfaces is that there's only so many combinations one can use - or expect the user to memorise.
2 years, 9 months ago on Ubuntu Wants to Replace Menus with a HUD