proposition: Lemma bin2' n: 'C(n.+1,2) * 2 = n * n.+1.
proof: Proof. by rewrite mulnC -mul_bin_diag bin1 mulnC. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype) mathcomp(ssrnat seq path div) mathcomp(fintype tuple finfun bigop finset binomial)