Proof using.
intros f g h n n' l.
generalize f g; clear f g.
induction l; auto.
intros.
simpl in *.
break_let.
destruct a.
find_injection.
set (f' := update2 _ _ _ _ _).
set (g' := update2 _ _ _ _ _).
rewrite (IHl f' g'); auto.
unfold f', g', update2.
break_if; auto.
break_and.
subst.
find_rewrite.
trivial.
Qed.

(* Specifications: *)
Lemma collate_in_in :
forall l h n n' (f : A -> A -> list B) a,
In a (f n' n) ->
In a ((collate A_eq_dec h f l) n' n).