proposition: Lemma F1a n: \sum_(i<n) 1 = n.
proof: Proof. by rewrite big_const_ord //; elim:n => // h Hrec; rewrite iterS Hrec add1n. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype) mathcomp(ssrnat seq path div) mathcomp(fintype tuple finfun bigop finset binomial)