// Multinomial expansion of power(+(x,...,z;0),k)*c^(n-k):
// Iterate over all partitions of k with exactly as many parts as
// there are symbolic terms in the basis (including zero parts).
// Multinomial expansion of power(+(x,...,z;0),k)*c^(n-k):
// Iterate over all partitions of k with exactly as many parts as
// there are symbolic terms in the basis (including zero parts).