proposition: Lemma rpredDr x y : x \in S -> (y + x \in S) = (y \in S).
proof: Proof. move=> Sx; apply/idP/idP=> [Sxy | /rpredD-> //]. by rewrite -(addrK x y) rpredB. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)