Prove that, for *N≥M≥n* and
(*N-M*)≥*n*,

where *C(a,b) = a*! /(*b*! (*a-b*)! ) is the binomial coefficient.

This is one of the basic identities concerning sum of products of
binomial coefficients: see the book *The Art of Computer Programming*
(by Prof. Knuth) eq. 1.2.6.21.

Marco Corvi - 2003