proposition: Lemma card_cnv e : #|e°| = #|e|.
proof: Proof. rewrite -(on_card_preimset (f := (fun x => (x.2,x.1)))). - apply: eq_card => [[x y]]. by rewrite !inE. - apply: onW_bij. apply: (Bijective (g := fun x => (x.2,x.1))); by case. Qed.
imports: Setoid Morphisms kat normalisation rewriting kat_tac comparisons rel relalg boolean mathcomp(all_ssreflect)