proposition: Lemma addqA: associative add.
proof: Proof. by move=> x y z; rewrite -[x]reprK -[y]reprK -[z]reprK !piE addrA. Qed.
imports: HB(structures) mathcomp(ssreflect eqtype choice ssreflect ssrbool ssrnat) mathcomp(ssrfun seq ssralg generic_quotient)