proposition: Lemma Dyck_path_size l: Dyck_path l -> size l = (DP_Tcount l).*2.
proof: Proof. by move => /andP [/eqP ha hb]; rewrite - DP_count - ha addnn. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype) mathcomp(ssrnat seq path div) mathcomp(fintype tuple finfun bigop finset binomial)