code: public void setXPathVariableResolver(XPathVariableResolver resolver) {
  
  		// resolver cannot be null
  		if (resolver == null) {
                      String fmsg = XSLMessages.createXPATHMessage(
                              XPATHErrorResources.ER_NULL_XPATH_VARIABLE_RESOLVER,
                              new Object[] {  CLASS_NAME } );
  		    throw new NullPointerException( fmsg );
  		}
  			
  		xPathVariableResolver = resolver;
  	}
docs: <p>Establish a default variable resolver.</p>
  
  <p>Any <code>XPath</code> objects constructed from this factory will use
  the specified resolver by default.</p>
  
  <p>A <code>NullPointerException</code> is thrown if <code>resolver</code> is <code>null</code>.</p>
  
  @param resolver Variable resolver.
  
  @throws NullPointerException If <code>resolver</code> is
  <code>null</code>.