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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 content_transfer_encoding(self) \ -> Optional[ContentTransferEncodingHeader]: """The ``Content-Transfer-Encoding`` header."""

        try:
            return cast(ContentTransferEncodingHeader,
                        self[b'content-transfer-encoding'][0])
        except (KeyError, IndexError):
            return None