Is a pos neg or zero?
It does not ask whether b is an integer, or if it's even or
I believe the correct behavior is:
if b is an even integer => pos
if b is an odd integer => neg
if b is not an integer, ask user about the sign of a^b
directly -- I suppose we could ask if b is rational, and if
so whether it is even/odd (pos), odd/odd (neg), or
odd/even (undefined -- is actually complex), but why
get into that mess?