checking method? return values

Someone on comp.lang.python pointed out that we could enforce the method
contract implied by "question" methods whose names end with a question mark
if we wanted to.  We could check all return values to make sure they were
either True or False.

The overhead would be a small amount at compile time and two compare
instructions at run time only for question methods.  There would be no
prototype lookup or anything expensive, just two direct compares.

Comments?  Votes?