Parse <doc_number> tag from `xml`.

    Args:
        xml (str): XML string returned from :func:`aleph.aleph.downloadRecords`

    Returns:
        str: Doc ID as string or "-1" if not found.

def getDocNumber(xml):
    """
    Parse <doc_number> tag from `xml`.

    Args:
        xml (str): XML string returned from :func:`aleph.aleph.downloadRecords`

    Returns:
        str: Doc ID as string or "-1" if not found.
    """
    dom = dhtmlparser.parseString(xml)

    doc_number_tag = dom.find("doc_number")

    if not doc_number_tag:
        return "-1"

    return doc_number_tag[0].getContent().strip()