proposition: Lemma Dyck_irrred_nnil l: Dyck_irred l -> ~~ nilp l.
proof: Proof. by move => /andP[/eqP -> _]. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype) mathcomp(ssrnat seq path div) mathcomp(fintype tuple finfun bigop finset binomial)