You should be able to raise integers to negative powers
mod n.  If b<0, pow(a,b,c)==pow(pow(a,-1,c),-b,c)
where pow(a,-1,c) is a's multiplicative inverse mod c,
computed with the extended Euclid algorithm.  This
would be in Python's spirit of math completeness and
would save people from the trouble of having to figure
out the algorithm over and over.  

I can come up with a patch for this if it's agreed on
as desirable.