Proof using.
induction l; intros; simpl in *.
- auto.
- repeat break_match; subst; simpl in *; auto.
break_if; try congruence.
Qed.

(* Specifications: *)
Lemma get_del_diff :
forall k k' l,
k <> k' ->
assoc (assoc_del l k') k = assoc l k.