code: def get_issns_for_journal(nlm_id):
      
      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]
docs: 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/