void insert_list (list_ref list, list_item item) {
   listnode_ref newnode = malloc (sizeof (struct listnode));
   assert (is_list (list));
   newnode->tag = listnode_tag;
   newnode->item = item;
   newnode->next = list->curr->next;
   newnode->prev = list->curr;
   list->curr->next = newnode;
   if (newnode->next != NULL)
      newnode->next->prev = newnode;
   else
      list->last = newnode;
   list->curr = newnode;
}