Options for undoing / reverting earlier commit

[Simon Harrison]

Let's say I have a repo with 100 commits.

For some reason I'm not happy with r50 and would like to remove *some*
of the changes that are in that revision, but keep all following
commits. What are my options to achieve this? I've been looking at
"revert" and "backout" but unless I'm mistaken, they don't offer an
interactive option (and may not be designed for this use case).

Alternatively, perhaps a patch could be generated and then unapplied?