Proof with congruence || auto.
induction T2; intros U k Fr; simpl in *; f_equal...
Case "typ_bvar".
destruct (k === n)... simpl. destruct (X == X)...
Case "typ_fvar".
destruct (a == X)... contradict Fr; fsetdec.
Qed.
(** The next lemma is a direct corollary of the immediately preceding lemma---the index is specialized to zero.  *)

Lemma subst_tt_intro : forall X T2 U,
X `notin` fv_tt T2 ->
open_tt T2 U = subst_tt X U (open_tt T2 X).