Can `__unittest = True`, which is used to hide stack frames from tracebacks inside the testing framework, be documented as being public, or, if that's not acceptable, be replaced by something that is? `_exc_info_to_string` is a hairy function to need to rewrite when authoring a testing framework, but it's a rather common thing to want, and would be nice if it was easily done by integrating with what `TestCase` is already doing.

It's also (with or without formal blessing) being used in the wild in at least one place I know of here http://bazaar.launchpad.net/~testtools-committers/testtools/trunk/view/head:/testtools/testcase.py#L798  and since it's not terribly critical and will fail rather nicely even if the API changes suddenly I'd bet other places are using it too, though I haven't bothered to check.

See also http://twistedmatrix.com/trac/ticket/4127  which would be easier to implement if this were acceptable to use for external code.