code: public DList concat(DList otherList)
      {
          DListImpl result = new DListImpl(pbKey);
          result.addAll(this);
          result.addAll(otherList);
          return result;
      }
docs: Creates a new <code>DList</code> object that contains the contents of this
  <code>DList</code> object concatenated
  with the contents of the <code>otherList</code> object.
  @param	otherList	The list whose elements are placed at the end of the list
  returned by this method.
  @return	A new <code>DList</code> that is the concatenation of this list and
  the list referenced by <code>otherList</code>.