proposition: Lemma extprod_mulgA : associative extprod_mulg.
proof: Proof. by move=> x y z; congr (_, _); apply: mulgA. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype bigop finset fingroup morphism) mathcomp(quotient action)