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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(input, fallback_encoding, errors='replace'):
 """ Decode a single string. :param input: A byte string :param fallback_encoding: An :class:`Encoding` object or a label string. The encoding to use if :obj:`input` does note have a BOM. :param errors: Type of error handling. See :func:`codecs.register`. :raises: :exc:`~exceptions.LookupError` for an unknown encoding label. :return: A ``(output, encoding)`` tuple of an Unicode string and an :obj:`Encoding`. """

    # Fail early if `encoding` is an invalid label.
    fallback_encoding = _get_encoding(fallback_encoding)
    bom_encoding, input = _detect_bom(input)
    encoding = bom_encoding or fallback_encoding
    return encoding.codec_info.decode(input, errors)[0], encoding