l: try solve [inversion H2].
  all: done.
Qed.


Definition irrelevant G D (a : tm) :=
  (forall x A, binds x (Tm A) G -> x `notin` fv_tm a) /\ Good G D.

Lemma irrelevant_Good : forall G D a, irrelevant G D a -> Good G D.
intros. inversion H.
auto.
Qed.
(*
Ltac impossible_defeq A B :=
  matc

impossible_defeq. - eapply invert_a_UAbs in H; eauto. destruct H as (A1 & B1 & H & _); eauto.