proposition: Lemma F_Sd1 : 'Fix_to[sd1] = [set x | coin1 x == coin3 x].
proof: Proof. apply/setP => x; rewrite (sameP afix1P eqP) !inE eqperm_map /=. rewrite /act_f sd1_inv !ffunE !permE /=. by rewrite !eqxx !andbT eq_sym /= andbb. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype tuple finfun bigop finset fingroup) mathcomp(action perm primitive_action)