code: public void setXPathFunctionResolver(XPathFunctionResolver resolver) {
          if ( resolver == null ) {
              String fmsg = XSLMessages.createXPATHMessage( 
                      XPATHErrorResources.ER_ARG_CANNOT_BE_NULL,
                      new Object[] {"XPathFunctionResolver"} );
              throw new NullPointerException( fmsg );
          }
          this.functionResolver = resolver;
      }
docs: <p>Establishes a function resolver.</p>
  
  @param resolver XPath function resolver