proposition: Lemma free_not0 v X : free X -> v \in X -> v != 0.
proof: Proof. by rewrite free_directv andbC => /andP[_ /memPn]; apply. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype bigop finfun tuple) mathcomp(ssralg matrix mxalgebra zmodp)