proposition: Lemma rcosetP A x y : reflect (exists2 a, a \in A & y = a * x) (y \in A :* x).
proof: Proof. by rewrite -rcosetE; apply: imsetP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)