Rather than factorial how about a product function, so factorial(n) =
product(xrange(1,n+1)).  I do like the idea of an imath module whether
or not it has factorial, and the topic of this rfe has drifted somewhat
towards the desirability of such a module, which is a reasonable
question for discussion.  I'm more or less indifferent to whether imath
has factorial in it or not.  I do think it would be useful for it to
have functions for things like binomial coefficients that were
implemented a little more cleverly than with large factorials, but in
this case factorial should be there too.