code: public CompositeHandler withUpdateHandler(final UpdateHandler handler) {
          if (handler == null) {
              throw new IllegalArgumentException();
          } // end of if
  
          // ---
  
          return new CompositeHandler(this.queryDetection,
                                      this.queryHandler,
                                      handler);
          
      }
docs: Returns a new handler based on this one,
  but with given update |handler| appended.
  
  @param handler Update handler
  @return a new composite handler with given update handler
  @throws IllegalArgumentException if handler is null