proposition: Lemma actNr x : {morph actr^~ x : u / - u}.
proof: Proof. by move=> u; apply: (addrI (u ^@ x)); rewrite -actAr !subrr act0r. 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)