proposition: Lemma index1g G H : H \subset G -> #|G : H| = 1%N -> H :=: G.
proof: Proof. by move=> sHG iHG; apply/eqP; rewrite eqEsubset sHG -indexg_eq1 iHG. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)