Hint Resolve tree_rep_nullval: saturate_local.

Lemma bst_left_entail: forall (t1 t1' t2: tree val) k (v p1 p2 p b: val),
Int.min_signed <= k <= Int.max_signed ->
is_pointer_or_null v ->
data_at Tsh (tptr t_struct_tree) p b *
data_at Tsh t_struct_tree (Vint (Int.repr k), (v, (p1, p2))) p *
tree_rep t1 p1 * tree_rep t2 p2
|-- treebox_rep t1 (field_address t_struct_tree [StructField _left] p) *
(treebox_rep t1'
(field_address t_struct_tree [StructField _left] p) -*
treebox_rep (T t1' k v t2) b).