proposition: Lemma class_sym x y : (x \in y ^: G) = (y \in x ^: G).
proof: Proof. by apply/idP/idP=> /class_eqP->. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)