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This seems like a pragmatic and sensible approach to take,
to me. It should probably be tested on the HP/UX boxes
(google for 'HP/UX testdrive')

I particularly like the idea of just putting a test in to
block readline in the non-main thread. It seems the pythonic
approach - since we can't guarantee behaviour that's
anything but sane, it seems like a plan. Or at least make it
issue a warning saying "don't do this" when readline is
invoked from a non-main thread.