proposition: Lemma fpredMr x y : x \in S -> x != 0 -> (y * x \in S) = (y \in S).
proof: Proof. by rewrite -!unitfE; apply: rpredMr. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)