def get_issns_for_journal(nlm_id):
    """Get a list of the ISSN numbers for a journal given its NLM ID.

    Information on NLM XML DTDs is available at
    https://www.nlm.nih.gov/databases/dtd/
    """
    params = {'db': 'nlmcatalog',
              'retmode': 'xml',
              'id': nlm_id}
    tree = send_request(pubmed_fetch, params)
    if tree is None:
        return None
    issn_list = tree.findall('.//ISSN')
    issn_linking = tree.findall('.//ISSNLinking')
    issns = issn_list + issn_linking
    # No ISSNs found!
    if not issns:
        return None
    else:
        return [issn.text for issn in issns]
Get a list of the ISSN numbers for a journal given its NLM ID.

    Information on NLM XML DTDs is available at
    https://www.nlm.nih.gov/databases/dtd/