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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 message(self, username, message_text):
 """Message an okcupid user. If an existing conversation between the logged in user and the target user can be found, reply to that thread instead of starting a new one. :param username: The username of the user to which the message should be sent. :type username: str :param message_text: The body of the message. :type message_text: str """

        # Try to reply to an existing thread.
        if not isinstance(username, six.string_types):
            username = username.username
        for mailbox in (self.inbox, self.outbox):
            for thread in mailbox:
                if thread.correspondent.lower() == username.lower():
                    thread.reply(message_text)
                    return

        return self._message_sender.send(username, message_text)