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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):
 """Returns a unicode string from a string, using UTF-8 to decode if needed. This function comes from `Tornado`_. :param value: A unicode or string to be decoded. :returns: The decoded string. """

  if isinstance(value, str):
    return value.decode('utf-8')

  assert isinstance(value, unicode)
  return value