proposition: Lemma bin2 n : 'C(n, 2) = (n * n.-1)./2.
proof: Proof. by rewrite -[n.-1]bin1 mul_bin_diag -divn2 mulKn. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div fintype tuple finfun bigop prime finset)