code: public void setXPathFunctionResolver(XPathFunctionResolver resolver) {
  			
              // resolver cannot be null
              if (resolver == null) {
                  String fmsg = XSLMessages.createXPATHMessage(
                          XPATHErrorResources.ER_NULL_XPATH_FUNCTION_RESOLVER,
                          new Object[] {  CLASS_NAME } );
                  throw new NullPointerException( fmsg );
              }
  			
              xPathFunctionResolver = resolver;
          }
docs: <p>Establish a default function 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 XPath function resolver.
  
  @throws NullPointerException If <code>resolver</code> is
  <code>null</code>.