Definition lseg_cell (ls: listspec list_structid list_link list_token)
(sh : share)
(v: elemtype ls) (x y: val) :=
!!is_pointer_or_null y && list_token sh x * list_cell ls sh v x * field_at sh list_struct (StructField list_link :: nil) (valinject (nested_field_type list_struct (StructField list_link :: nil)) y) x.

Lemma list_append_null:
forall
(ls: listspec list_structid list_link list_token)
(sh: share)
(hd mid: val) ct1 ct2,
lseg ls sh ct1 hd mid * lseg ls sh ct2 mid nullval |--
lseg ls sh (ct1++ct2) hd nullval.