intros. induction G. simpl in H. inversion H.
destruct a0 as [x0 s0].
simpl in H.
apply binds_cons_iff in H. destruct H. destruct H. subst.
exists s0. auto.
apply IHG in H. destruct H as [s' [ EQ B]].
exists s'. auto.
Qed.
(* -------------------------------------- *)

(* Specifications: *)
Lemma binds_cases: forall G F x A y B,
uniq (F ++ [(y, B)] ++ G) ->
@binds sort x A (F ++ [(y, B)] ++ G) ->
(binds x A F /\ x <> y /\ x `notin` dom G) \/ (x = y /\ A = B) \/ (binds x A G /\ x <> y /\ x `notin` dom F).