"U" does not do anything

Originally reported by: **Anonymous**

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I'm trying to learn Vim keybindings from vimtutor, and noticed that "U" does not undo all changes on a line. Is this not bound for a reason, or is this a missing feature? I don't seem to see any mention of it anywhere online.

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- Bitbucket: https://bitbucket.org/lyro/evil/issue/144

Start by reproducing the behavior of "U" while following vimtutor, then inspect the Evil keybinding implementation for the relevant undo behavior. Done means deciding whether "U" should undo all changes on the current line and, if so, adding the corresponding implementation and test coverage.

Written by the indexing model from the issue text.