diff_sorted_lists (GList *list1,
                   GList *list2,
                   GCompareFunc compare,
                   GList **added,
                   GList **removed)
{
  int order;

  *added = *removed = NULL;

  while (list1 != NULL && list2 != NULL)
    {
      order = (*compare) (list1->data, list2->data);
      if (order < 0)
        {
          *removed = g_list_prepend (*removed, list1->data);
          list1 = list1->next;
        }
      else if (order > 0)
        {
          *added = g_list_prepend (*added, list2->data);
          list2 = list2->next;
        }
      else
        { /* same item */
          list1 = list1->next;
          list2 = list2->next;
        }
    }

  while (list1 != NULL)
    {
      *removed = g_list_prepend (*removed, list1->data);
      list1 = list1->next;
    }
  while (list2 != NULL)
    {
      *added = g_list_prepend (*added, list2->data);
      list2 = list2->next;
    }
}

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