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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 decode_tag(stream):
 """Decode a tag value from a serialized stream. :param stream: Source data stream :type stream: io.BytesIO :returns: Decoded tag :rtype: bytes """

    (reserved, tag) = unpack_value(">cc", stream)

    if reserved != b"\x00":
        raise DeserializationError("Invalid tag: reserved byte is not null")

    return tag