code: public void addSelector( SelectorName first,
                               SelectorName second ) {
          if (first != null) selectors.add(first);
          if (second != null) selectors.add(second);
      }
docs: Add the selectors to this plan node. This method does nothing for any supplied selector that is null.
  
  @param first the first symbol to be added
  @param second the second symbol to be added