def METHOD_NAME(self, doc):
    """
    Map the XML input into an RTEPair.
    This uses the ``getiterator()`` method from the ElementTree package to
    find all the ``<pair>`` elements.
    :param doc: a parsed XML document
    :rtype: list(RTEPair)
    """
    try:
        challenge = doc.attrib["challenge"]
    except KeyError:
        challenge = None
    pairiter = doc.iter("pair")
    return [RTEPair(pair, challenge=challenge) for pair in pairiter]