def parse_pmid(medline):
    """Parse PMID from article

    Parameters
    ----------
    medline: Element
        The lxml node pointing to a medline document

    Returns
    -------
    pmid: str
        String version of the PubMed ID
    """
    if medline.find('PMID') is not None:
        pmid = medline.find('PMID').text
    else:
        pmid = ''
    return pmid