code: protected T popBackEntry() {
          if (last == null) {
              return null;
          }
          T prev = last.getPrev();
          if (prev != null) {
              prev.setNext(null);
          } else {
              head = null;
          }
          T e = last;
          last = prev;
          e.setPrev(null);
          size--;
          return e;
      }
docs: Removes and returns the last entry in the list. If the list is empty, it remains unmodified and <code>null</code>
  is returned.
  
  @return the previously first entry in the list, or <code>null</code>.