Hint Resolve tree_rep_nullval: saturate_local.

Lemma tree_rep_internal: forall l x v r (p p1 p2: val),
Int.min_signed <= Z.of_nat x <= Int.max_signed ->
tc_val (tptr Tvoid) v ->
data_at Tsh t_struct_tree (Vint (Int.repr (Z.of_nat x)), (v, (p1, p2))) p *
tree_rep l p1 *
tree_rep r p2 |--
tree_rep (T l x v r) p.