proposition: Lemma catl_free Y X : free (X ++ Y) -> free X.
proof: Proof. by rewrite cat_free => /and3P[]. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype bigop finfun tuple) mathcomp(ssralg matrix mxalgebra zmodp)