code: Node<E> back() {
              Node<E> f = predecessor();
              return (f == null || f.isSpecial()) ? null : f;
          }
docs: Returns previous node containing a nondeleted user element, if
  possible.  Use for backward list traversal, but beware that
  if this method is called from a deleted node, it might not
  be able to determine a usable predecessor.
  
  @return predecessor, or null if no such could be found