Design review: Pmf arithmetic

[AllenDowney]

When you apply an arithmetic operation to a Pmf, there are two things you might mean. For example, multiplication might mean that you want to multiply the probabilities by a scalar, sequence, or sequence of probabilities. Or it might mean you want to compute the Pmf of the product of the quantities.

One possibility is to use the arithmetic operators for the first kind of operation and the named methods (add, sub, mul, div, etc.) for the second kind.

Thoughts?