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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 mention(self):
 """ Get mention if a Chat has a username, or get full name if this is a Private Chat, otherwise None is returned """

        if self.username:
            return '@' + self.username
        if self.type == ChatType.PRIVATE:
            return self.full_name
        return None