proposition: Lemma fhrel_card1 : #|fone aT| = #|aT|.
proof: Proof. rewrite -(on_card_preimset (f := (fun x => (x,x)))). - rewrite eq_cardT ?cardT //= => x. by rewrite !inE oneE eqxx. - exists fst => //= [[x y]]. by rewrite inE oneE /= => /eqP->. Qed.
imports: Setoid Morphisms kat normalisation rewriting kat_tac comparisons rel relalg boolean mathcomp(all_ssreflect)