proposition: Lemma envelop_mx_id x : x \in G -> (rG x \in E_G)%MS.
proof: Proof. by move=> Gx; rewrite (eq_row_sub (enum_rank_in Gx x)) // rowK enum_rankK_in. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div choice fintype tuple finfun bigop prime) mathcomp(ssralg poly polydiv finset fingroup morphism) mathcomp(perm automorphism quotient finalg action zmodp) mathcomp(commutator cyclic center pgroup matrix mxalgebra) mathcomp(mxpoly)