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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def add_message_listener(self, name, fn):
 """ Adds a message listener function that will be called every time the specified message is received. .. tip:: We recommend you use :py:func:`on_message` instead of this method as it has a more elegant syntax. This method is only preferred if you need to be able to :py:func:`remove the listener <remove_message_listener>`. The callback function must have three arguments: * ``self`` - the current vehicle. * ``name`` - the name of the message that was intercepted. * ``message`` - the actual message (a `pymavlink <http://www.qgroundcontrol.org/mavlink/pymavlink>`_ `class <https://www.samba.org/tridge/UAV/pymavlink/apidocs/classIndex.html>`_). For example, in the fragment below ``my_method`` will be called for every heartbeat message: .. code:: python #Callback method for new messages def my_method(self, name, msg):
 pass vehicle.add_message_listener('HEARTBEAT',my_method) See :ref:`mavlink_messages` for more information. :param String name: The name of the message to be intercepted by the listener function (or '*' to get all messages). :param fn: The listener function that will be called if a message is received. """

        name = str(name)
        if name not in self._message_listeners:
            self._message_listeners[name] = []
        if fn not in self._message_listeners[name]:
            self._message_listeners[name].append(fn)