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