Print a traceback from an extension?

[gb at cs.unc.edu]

Can I print a traceback from within a extension written in C? 

I know we're supposed to just keep returning NULL until we make it
back to the main loop. That normally works fine. 

But I'm calling Python code from a callback that is called by C
code. The Python code fails so PyEval_CallObject returns NULL. I want
to provide a helpful message at that point.

Can this be done? If so, can someone suggest where to look for an
example?

Thanks
gb