RingDistrMPoly implements a ring of distributed multivariate
polynomials: you may think of the elements as being ordered lists of
coefficient and power product pairs (with the additional guarantee
that the coefficients are non-zero, and that the power products are
all distinct.
A RingDistrMPoly is a concrete instance of a SparsePolyRing.
I have implemented HomImpl and IdealImpl as private subclasses since
their existence should not be known outside the scope of the class
RingDistrMPolyImpl.
Documentation almost completely absent.
The implementation of RingDistrMPolyImpl::HomImpl::myApply is very
poor. I simply needed some easy code that would work. A major
overhaul will be needed when I have understood how best to implement
it.