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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 get_message_type_for_app(app_name, default_message_type_alias):
 """Returns a registered message type object for a given application. Supposed to be used by reusable applications authors, to get message type objects which may be overridden by project authors using `override_message_type_for_app`. :param str|unicode app_name: :param str|unicode default_message_type_alias: :return: a message type object overridden is so, or the default :rtype: MessageBase """

    message_type = default_message_type_alias
    try:
        message_type = _MESSAGES_FOR_APPS[app_name][message_type]
    except KeyError:
        pass
    return get_registered_message_type(message_type)