:meth:`WMessengerOnionSessionFlowProto.iterator` implementation

def iterator(self, envelope):
		""" :meth:`WMessengerOnionSessionFlowProto.iterator` implementation
		"""
		for pair in self.__pairs:
			if pair.comparator().match(envelope) is True:
				return pair.flow().iterator(envelope)

		if self.__default_flow is not None:
			return self.__default_flow.iterator(envelope)