proposition: Lemma bin_sub n m : m <= n -> 'C(n, n - m) = 'C(n, m).
proof: Proof. by move=> le_m_n; rewrite !bin_ffactd !ffact_factd ?leq_subr // divnAC subKn. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div fintype tuple finfun bigop prime finset)