Theorem 1.1 If a operation can be performed in n1
ways, and if
for each of these a second operation can be performed in n2
ways,
and for each of the first two a third operation can be performed in
n3
ways and so forth, then the sequence of k operations can be
performed in n1
n2
n3… nk ways (Product Rule)
Junsawang, P. 316207