proposition: Lemma subtype_trans: forall t1 t2 t3, t1 <: t2 -> t2 <: t3 -> t1 <: t3.
proof: Proof. intros t1 t2; revert t2 t1. induction t2; intros t1 t3 S1 S2; inversion S1; inversion S2; subst; eauto using subtype. Qed.
imports: Coq(Bool String List Program.Equality) CDF(Sequences)