induction i; intuition; simpl in *.
intuition.
intuition.
Qed.

Lemma listReplace_in :
forall (A : Set)(ls : list A)(a1 a2 def : A)(i : nat),
In a1 (listReplace ls i a2 def) ->
(In a1 ls \/ a1 = a2 \/ a1 = def).