docs: def references2marc(self, key, value):
      """Populate the ``999C5`` MARC field."""
      reference = value.get('reference', {})
  
      pids = force_list(reference.get('persistent_identifiers'))
      a_values = ['doi:' + el for el in force_list(reference.get('dois'))]
      a_values.extend(['hdl:' + el['value'] for el in pids if el.get('schema') == 'HDL'])
      a_values.extend(['urn:' + el['value'] for el in pids if el.get('schema') == 'URN'])
  
      external_ids = force_list(reference.get('external_system_identifiers'))
      u_values = force_list(get_value(reference, 'urls.value'))
      u_values.extend(CDS_RECORD_FORMAT.format(el['value']) for el in external_ids if el.get('schema') == 'CDS')
      u_values.extend(ADS_RECORD_FORMAT.format(el['value']) for el in external_ids if el.get('schema') == 'ADS')
  
      authors = force_list(reference.get('authors'))
      e_values = [el['full_name'] for el in authors if el.get('inspire_role') == 'editor']
      h_values = [el['full_name'] for el in authors if el.get('inspire_role') != 'editor']
  
      r_values = force_list(reference.get('report_numbers'))
      if reference.get('arxiv_eprint'):
          arxiv_eprint = reference['arxiv_eprint']
          r_values.append('arXiv:' + arxiv_eprint if is_arxiv_post_2007(arxiv_eprint) else arxiv_eprint)
  
      if reference.get('publication_info'):
          reference['publication_info'] = convert_new_publication_info_to_old([reference['publication_info']])[0]
      journal_title = get_value(reference, 'publication_info.journal_title')
      journal_volume = get_value(reference, 'publication_info.journal_volume')
      page_start = get_value(reference, 'publication_info.page_start')
      page_end = get_value(reference, 'publication_info.page_end')
      artid = get_value(reference, 'publication_info.artid')
      s_value = build_pubnote(journal_title, journal_volume, page_start, page_end, artid)
  
      m_value = ' / '.join(force_list(reference.get('misc')))
  
      return {
          '0': get_recid_from_ref(value.get('record')),
          '9': 'CURATOR' if value.get('legacy_curated') else None,
          'a': a_values,
          'b': get_value(reference, 'publication_info.cnum'),
          'c': reference.get('collaborations'),
          'e': e_values,
          'h': h_values,
          'i': reference.get('isbn'),
          'k': reference.get('texkey'),
          'm': m_value,
          'o': reference.get('label'),
          'p': get_value(reference, 'imprint.publisher'),
          'q': get_value(reference, 'publication_info.parent_title'),
          'r': r_values,
          's': s_value,
          't': get_value(reference, 'title.title'),
          'u': u_values,
          'x': get_value(value, 'raw_refs.value'),
          'y': get_value(reference, 'publication_info.year'),
          'z': 1 if value.get('curated_relation') else 0,
      }