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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 post_message(plugin, polled_time, identity, message):
 """Post single message :type plugin: errbot.BotPlugin :type polled_time: datetime.datetime :type identity: str :type message: str """

    user = plugin.build_identifier(identity)
    return plugin.send(user, message)