Unwanted undo-boundary

[Lennart Borgman]

I just got a bug report saying that a single indent-region in mumamo
can't be undone with a single undo command. (This is with the latest
nXhtml beta and I can confirm it with an Emacs from 2010-02-03.)

I assume this means that something is calling undo-boundary, but I
can't find out what. There is nothing in nXhtml that does it. (Yes,
one function does, but it is not involved here.) I tried to trace
undo-boundary, but I get no hit.

Does anyone have a good idea for how to find out what is happening? Am
I wrong assuming that undo-boundary have been called (implicitly or
explicitly)?