proposition: Lemma F5 n: (\sum_(i<n.+1) i) * 2 = n * (n.+1).
proof: Proof. by rewrite F5aux bin2'. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype) mathcomp(ssrnat seq path div) mathcomp(fintype tuple finfun bigop finset binomial)