proposition: Lemma T2lt_psi a b a' b': [a,b] < [a', b'] = lt_psi a b a' b'.
proof: Proof. by rewrite {1} T2ltE /lt_rec ltnn if_same if_simpl. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(fintype bigop)