proposition: Lemma subring_closedB : subring_closed -> zmod_closed S.
proof: Proof. by case=> S1 SB _; split; rewrite // -(subrr 1) SB. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)