@staticmethod
def _get_docid(node):
    """Try to get a missing ID, preferring PMCID and PMID."""
    for t in ('="pmc"', '="pubmed"', ''):
        n = node.find('.//article-id[@pub-id-type{}]'.format(t))
        if n is not None:
            return n.text
    return 'unknown'
