void delete_list (list_ref list) {
   listnode_ref tempnode = 0;
   assert (is_list (list));
   assert (! empty_list (list));
   tempnode = list->curr;
   if (list->curr->next != NULL){
      list->curr->prev = tempnode->next;
      list->curr = tempnode->next;
   }else{
      list->last->prev = tempnode->prev;
      list->curr = list->last;
   }
   free (tempnode);
}