Proof using.
intros.
rewrite collate_app.
revert f.
induction l2; intros; simpl in *; auto.
break_let.
subst_max.
simpl in *.
assert (H_neq: a0 <> h'); auto.
rewrite collate_neq_update2; auto.
Qed.

Lemma collate_not_in_mid :
forall h h' l1 l2 (f : A -> A -> list B) m,
~ In h (map fst (l1 ++ l2)) ->
collate A_eq_dec h' (update2 A_eq_dec f h' h (f h' h ++ [m])) (l1 ++ l2) = collate A_eq_dec h' f (l1 ++ (h, m) :: l2).