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The inspect isSOMETHING() functions all return True or False, except 
for isgeneratorfunction(), which returns True or None.
The body of the function is very brief:
if (isfunction(object) or ismethod(object)) and \
 object.func_code.co_flags & CO_GENERATOR:
 return True
The behaviour can be made consistent with the other routines by either 
appending "else: return False", or changing the body to:
return bool(
 (isfunction(object) or ismethod(object)) and
 object.func_code.co_flags & CO_GENERATOR)