proposition: Lemma cfIsomKV : cancel (cfIsom (isom_sym isoGR)) (cfIsom isoGR).
proof: Proof. move=> phi; apply/cfun_inP=> y Ry; pose injGR := isom_inj isoGR. rewrite -{1}[y](invmK injGR) ?(isom_im isoGR) //. suffices /morphpreP[fGy Gf'y]: y \in invm injGR @*^-1 G by rewrite !cfIsomE. by rewrite morphpre_invm (isom_im isoGR). Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div choice fintype tuple finfun bigop prime order) mathcomp(ssralg poly finset fingroup morphism perm) mathcomp(automorphism quotient finalg action gproduct) mathcomp(zmodp commutator cyclic center pgroup sylow) mathcomp(matrix vector falgebra ssrnum algC algnum) mathcomp(archimedean)