proposition: Lemma order_dvdG G a : a \in G -> #[a] %| #|G|.
proof: Proof. by move=> Ga; apply: cardSg; rewrite cycle_subG. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice div) mathcomp(fintype bigop prime finset fingroup morphism) mathcomp(perm automorphism quotient gproduct ssralg) mathcomp(finalg zmodp poly)