(****************)
intros. rewrite trunc_app. rewrite (trunc_max v1 (length v1 + i)).
replace (length v1 + i - length v1) with i. trivial with arith. auto with arith.
auto with arith.
Qed.

(* Specifications: *)
Lemma trunc_eq :
forall (v1 v2 : list A) (i : nat), v1 = v2 -> trunc v1 i = trunc v2 i.