slist_get0
if (slist_add(list, slist_get0(add_items, i)) == NULL)
return slist_get0(list, idx);
p = slist_get0(list, (j + i) / 2);
while (compar(slist_get0(list, i), p) < 0)
while (compar(slist_get0(list, j), p) > 0)