proposition: Fact nat_num_semiring : semiring_closed nat_num.
proof: Proof. by do 2![split] => //= _ _ /natrP[n ->] /natrP[m ->]; rewrite -(natrD, natrM). Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)