proposition: Lemma rpredBl x y : x \in S -> (x - y \in S) = (y \in S).
proof: Proof. by rewrite -(rpredN _ y); apply: rpredDl. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)