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

Lemma get_set_diff_default :
forall (k k' : K) (v : V) l d,
k <> k' ->
assoc_default (assoc_set l k v) k' d = assoc_default l k' d.