proposition: Lemma divr_closedV : divr_closed -> invr_closed.
proof: Proof. by case=> S1 Sdiv x Sx; rewrite -[x^-1]mul1r Sdiv. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)