proposition: Lemma perm_on1 H : perm_on H 1.
proof: Proof. by apply/subsetP=> x; rewrite inE /= perm1 eqxx. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(choice fintype tuple finfun bigop finset binomial) mathcomp(fingroup morphism)