proposition: Lemma actrKV x : cancel (actr^~ x^-1%g) (actr^~ x).
proof: Proof. by move=> u; rewrite /= -{2}(invgK x) actrK. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq path) mathcomp(div choice fintype bigop ssralg finset fingroup) mathcomp(morphism perm finalg action gproduct commutator) mathcomp(cyclic)