proposition: Lemma Dyck_modify_size l: size (Dyck_modify l) = size l.
proof: Proof. rewrite -{2}(DP_splita_recover l) /(Dyck_modify) !size_cat. by rewrite swap_but_last_size. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype) mathcomp(ssrnat seq path div) mathcomp(fintype tuple finfun bigop finset binomial)