Polynomials in a countably infinite set of variables x1, x2, x3, ...
A polynomial is just a list of monomials, construed as their sum. We maintain the invariant that polynomials are always sorted by the ordering on monomials defined in MathObj.Monomial: first by partition degree, then by largest variable index (decreasing), then by exponent of the highest-index variable (decreasing). This works out nicely for operations on cycle index series.
Instances are provided for Additive, Ring, Differential (partial differentiation with respect to x1), and Show.