proposition: Lemma divring_closedBM : divring_closed -> subring_closed S.
proof: Proof. by case=> S1 SB Sdiv; split=> //; case: divr_closedM. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)