proposition: Lemma truncM : {in nat_num &, {morph trunc : x y / x * y >-> (x * y)%N}}.
proof: Proof. by move=> _ _ /natrP[n1 ->] /natrP[n2 ->]; rewrite -natrM !natrK. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype bigop order ssralg poly ssrnum ssrint)