Fixpoint gamma_sub (G G':etype) {struct G} : Prop :=
match G, G' with
| nil, nil => True
| x::xs, y::ys => TT |-- y >=> x /\ gamma_sub xs ys
| _, _ => False
end.

Lemma T_Abs: forall G sigma e tau,
Typ (sigma :: G) e tau ->
Typ G (Lam e) (ty_lam sigma tau).