Comparing complex numbers with cmp yields:

>>> cmp(1+3j, 1+3j)
0
>>> cmp(1+3j, 3+4j)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: cannot compare complex numbers using <, <=,
>, >=

Well, I didn't use <, <=, > or >=. It's not a major
bug, but it doesn't look too nice... would it be
possible to return NotImplemented? Or would that be
semantically incorrect?