proposition: Lemma le_succ_succE x y: T2nf x -> T2nf y -> (T2succ x <= T2succ y) = (x <= y).
proof: Proof. by move => na nb; rewrite /T2le (succ_inj na nb) (lt_succ_succE na nb). Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(fintype bigop)