new_contents: from abc import ABCMeta, abstractmethod, abstractproperty
  from flow.protocol.exceptions import InvalidMessageException
  from flow.interfaces import IHandler
  from twisted.internet import defer
  import logging
  
  LOG = logging.getLogger(__name__)
  
  class Handler(IHandler):
      __metaclass__ = ABCMeta
  
      def __call__(self, message):
          try:
              deferred = self._handle_message(message)
          except InvalidMessageException:
              LOG.exception('Invalid message.  message = %s', message)
              deferred = defer.fail(None)
          except:
              LOG.exception('Unexpected error handling message. message = %s',
                      message)
              deferred = defer.fail(None)
          return deferred
  
      @abstractmethod
      def _handle_message(self, message):
          """
          Returns a deferred that will callback when the message has been
          completely handled, or will errback when the message cannot be
          handled.  This function may also raise an InvalidMessageException.
          """
  
subject: FIX add the handler previously mentioned