code: public XObject execute(XPathContext xctxt) throws javax.xml.transform.TransformerException
    {
  
      // Programmer's assert.  (And, no, I don't want the method to be abstract).
      System.out.println("Error! Function.execute should not be called!");
  
      return null;
    }
docs: Execute an XPath function object.  The function must return
  a valid object.
  @param xctxt The execution current context.
  @return A valid XObject.
  
  @throws javax.xml.transform.TransformerException