We characterize which coefficients in the expansion of a certain product are nonzero. This answers a question J. D'Angelo from the field of CR geometry. In the process of giving our answer, we explain how the set of permutations w in S_p for which all of the differences (w(i)-i) mod p are 0, 1 or q relates to state of the art computer graphics from 30 years ago. This is joint work with N. Loehr and H. Wilf.