Note that the selection of functions is similar, but not identical, to
that in module math. The reason for having
two modules is that some users aren't interested in complex numbers,
and perhaps don't even know what they are. They would rather have
math.sqrt(-1) raise an exception than return a complex number.
Also note that the functions defined in cmath always return a
complex number, even if the answer can be expressed as a real number
(in which case the complex number has an imaginary part of zero).