code: def sequencebank(self):
          
          path = ['enhancement', 'sequencebanks', 'sequencebank']
          items = listify(chained_get(self._head, path, []))
          bank = namedtuple('Sequencebank', 'name sequence_number type')
          out = []
          for item in items:
              numbers = listify(item['sequence-number'])
              for number in numbers:
                  new = bank(name=item['@name'], sequence_number=number['$'],
                             type=number['@type'])
                  out.append(new)
          return out or None
docs: List of namedtuples representing biological entities defined or
          mentioned in the text, in the form (name, sequence_number, type).