proposition: Lemma rpredV x : (x^-1 \in S) = (x \in S).
proof: Proof. by apply/idP/idP=> /rpredVr; rewrite ?invrK. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)