code: public XObject execute(XPathContext xctxt) throws javax.xml.transform.TransformerException
    {
  
      DTMIterator nodes = m_arg0.asIterator(xctxt, xctxt.getCurrentNode());
      double sum = 0.0;
      int pos;
  
      while (DTM.NULL != (pos = nodes.nextNode()))
      {
        DTM dtm = nodes.getDTM(pos);
        XMLString s = dtm.getStringValue(pos);
  
        if (null != s)
          sum += s.toDouble();
      }
      nodes.detach();
  
      return new XNumber(sum);
    }
docs: Execute the function.  The function must return
  a valid object.
  @param xctxt The current execution context.
  @return A valid XObject.
  
  @throws javax.xml.transform.TransformerException