proposition: Lemma DP_irred_size2 l: Dyck_irred l -> size l == 2 -> l = DyckFT.
proof: Proof. move => /andP [/eqP -> _]; case:(DP_lower l) => // a l'. by rewrite /DP_raise /= size_rcons //. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype) mathcomp(ssrnat seq path div) mathcomp(fintype tuple finfun bigop finset binomial)