  The diamond product is the poset operation that when applied to the face
lattices of two polytopes results in the face lattice of the Cartesian product
of the polytopes. Application of the diamond product to two Eulerian posets is
a bilinear operation on the cd-indices of the two posets, yielding a product on
cd-polynomials. A lattice path interpretation is provided for this product of
two cd-monomials.
