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