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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 to_unicode(value):
 """ Converts a string argument to a unicode string. If the argument is already a unicode string or None, it is returned unchanged. Otherwise it must be a byte string and is decoded as utf8. """

    if isinstance(value, _TO_UNICODE_TYPES):
        return value
    if not isinstance(value, bytes):
        raise TypeError(
            "Expected bytes, unicode, or None; got %r" % type(value))
    return value.decode("utf-8")