proposition: Lemma rstab_quo m (U : 'M_(m, n)) : rstab rGH U = (rstab rG U / H)%g.
proof: Proof. apply/setP=> Hx /[!inE]; apply/andP/idP=> [[]|]; case/morphimP=> x Nx Gx ->{Hx}. by rewrite quo_repr_coset // => nUx; rewrite mem_morphim // inE Gx. by case/setIdP: Gx => Gx nUx; rewrite quo_repr_coset ?mem_morphim. 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)