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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 of_cte(cls, header: Optional[ContentTransferEncodingHeader]) \ -> 'MessageDecoder': """Return a decoder from the CTE header value. There is built-in support for ``7bit``, ``8bit``, ``quoted-printable``, and ``base64`` CTE header values. Decoders can be added or overridden with the :attr:`.registry` dictionary. Args: header: The CTE header value. """

        if header is None:
            return _NoopDecoder()
        hdr_str = str(header).lower()
        custom = cls.registry.get(hdr_str)
        if custom is not None:
            return custom
        elif hdr_str in ('7bit', '8bit'):
            return _NoopDecoder()
        elif hdr_str == 'quoted-printable':
            return _QuotedPrintableDecoder()
        elif hdr_str == 'base64':
            return _Base64Decoder()
        else:
            raise NotImplementedError(hdr_str)