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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 disk_xml(identifier, pool_xml, base_volume_xml, cow):
 """Clones volume_xml updating the required fields. * name * target path * backingStore """

    pool = etree.fromstring(pool_xml)
    base_volume = etree.fromstring(base_volume_xml)
    pool_path = pool.find('.//path').text
    base_path = base_volume.find('.//target/path').text
    target_path = os.path.join(pool_path, '%s.qcow2' % identifier)
    volume_xml = VOLUME_DEFAULT_CONFIG.format(identifier, target_path)
    volume = etree.fromstring(volume_xml)
    base_volume_capacity = base_volume.find(".//capacity")

    volume.append(base_volume_capacity)

    if cow:
        backing_xml = BACKING_STORE_DEFAULT_CONFIG.format(base_path)
        backing_store = etree.fromstring(backing_xml)
        volume.append(backing_store)

    return etree.tostring(volume).decode('utf-8')