Proof using.
intros c a l; generalize a l; elim c; clear a l c; simpl in |- *; auto.
intros a l H H0; case H0.
intros a; case a.
intros a0 l l0 H a1 l1 H0 H1; case (eqA_dec a1 a0).
intros H2; case H1; intros H3.
injection H3; auto.
apply unique_key_in_inv with (l := (a0, l) :: l0) (a := a0); simpl in |- *;
auto.
rewrite <- H2; auto.
intros H2; case H1; auto.
intros H3; case H2; injection H3; auto.
intros; apply H; auto.
apply unique_key_inv with (1 := H0); auto.
Qed.

(* There is not more than what is in the code *)
Theorem not_in_find_code :
forall a l, (forall p, ~ In (a, p) l) -> find_code a l = nil.