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_Var: forall G x tau,
nth_error G x = Some tau -> (* g(x) = tau *)
Typ G (Var x) tau.