proposition: Lemma extprod_mul1g : left_id (1, 1) extprod_mulg.
proof: Proof. by case=> x1 x2; congr (_, _); apply: mul1g. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype bigop finset fingroup morphism) mathcomp(quotient action)