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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 _read_marcxml(xml):
 """ Read MARC XML or OAI file, convert, add namespace and return XML in required format with all necessities. Args: xml (str):
 Filename or XML string. Don't use ``\\n`` in case of filename. Returns: obj: Required XML parsed with ``lxml.etree``. """

    # read file, if `xml` is valid file path
    marc_xml = _read_content_or_path(xml)


    # process input file - convert it from possible OAI to MARC XML and add
    # required XML namespaces
    marc_xml = _oai_to_xml(marc_xml)
    marc_xml = _add_namespace(marc_xml)

    file_obj = StringIO.StringIO(marc_xml)

    return ET.parse(file_obj)