proposition: Lemma mul_bin_left n m : m.+1 * 'C(n, m.+1) = (n - m) * 'C(n, m).
proof: Proof. by rewrite -mul_bin_diag mul_bin_down. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div fintype tuple finfun bigop prime finset)