proposition: Lemma class_eqP x y : reflect (x ^: G = y ^: G) (x \in y ^: G).
proof: Proof. by apply: (iffP idP) => [/imsetP[z Gz ->] | <-]; rewrite ?class_refl ?classGidl. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)