How to get rid of a bad branch merge?

[Steven]

Say I have 3 branches, A, B, and C

C contains merges from branch A, and A contains merges from C.

If someone accidentally merge B into A, we can no longer merge A and C
because they will have to essentially merge part of B and C.  In addition,
if we merge A into C and use C's version when resolving conflicts with B,
if someone else tries to merge B into C later on, mercurial will notice
part of B is already merged into C, and ignore those changes.

Is there a way to remove that troublesome merge?  I tried creating a new
branch and rebasing (have to redo all the merges in A again), hg prune
(evolve doesn't support merges yet), graft/transplant (doesn't really work
with merges).

Thanks.
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