proposition: Lemma mulzC : commutative mulz.
proof: Proof. by move=> [] m [] n //=; rewrite mulnC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat choice seq) mathcomp(fintype finfun bigop order ssralg countalg ssrnum) mathcomp(poly)