proposition: Lemma extprod_mulVg : left_inverse (1, 1) extprod_invg extprod_mulg.
proof: Proof. by move=> x; congr (_, _); apply: mulVg. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype bigop finset fingroup morphism) mathcomp(quotient action)