'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)'

def _read_etree(self, doc):

    try:
        challenge = doc.attrib[u'challenge']
    except KeyError:
        challenge = None
    return [RTEPair(pair, challenge=challenge) for pair in doc.getiterator(u'pair')]
