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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 _RemoveIllegalXMLCharacters(self, xml_string):
 """Removes illegal characters for XML. If the input is not a string it will be returned unchanged. Args: xml_string (str):
 XML with possible illegal characters. Returns: str: XML where all illegal characters have been removed. """

    if not isinstance(xml_string, py2to3.STRING_TYPES):
      return xml_string

    return self._ILLEGAL_XML_RE.sub('\ufffd', xml_string)