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