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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 sec(self, is_compressed=None):
 """ Return the SEC representation of this key, if available. """

        if is_compressed is None:
            is_compressed = self.is_compressed()
        public_pair = self.public_pair()
        if public_pair is None:
            return None
        return public_pair_to_sec(public_pair, compressed=is_compressed)