proposition: Lemma DP_ph1_size2 l: DP_ph1_aux2 l -> size l == 2 -> l = DyckFT.
proof: Proof. move /DP_ph1_Dyck_aux. by case:l => // b [ // | a [ | c l //]]; case:a; case: b. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype) mathcomp(ssrnat seq path div) mathcomp(fintype tuple finfun bigop finset binomial)