flags: allow specifying --no-boolean-flag on the command line (BC) This makes it much easier to enable some anti-foot-shooting features (like update --check) by default, because now all boolean flags can be explicitly disabled on the command line without having to use HGPLAIN or similar. Flags which don't deserve this treatment can be removed from consideration by adding them to the nevernegate set in fancyopts. This doesn't make it any easier to identify when a flag is set: opts still always gets filled in, either with the user-specified flag value or with the default from the flags list in the command table. Improving that would probably clean things up a bit, but for now if you want a boolean flag and care if it was explicitly false or default false (or true, but nobody uses that functionality because before now it was nonsense) you need to use None as your default rather than True or False. This doesn't (yet) update help output, because I'm not quite sure how to do that cleanly.