def handler ( method )
      raise TypeError,  error_message(method, __method__) unless method.instance_of? Method
      @handler = method
    end
def handler ( method )
      raise TypeError,  error_message(method, __method__) unless method.instance_of? Method
      @handler = method
    end
call-seq:
   handler( method(:some_handler_method) )

 Set handler method for current state.