docs: def as_conll(self):
          """Represent this Token as a line as a string in CoNLL-X format."""
          def get(field):
              value = getattr(self, field)
              if value is None:
                  value = '_'
              elif field == 'feats':
                  value = '|'.join(value)
              return str(value)
          return '\t'.join([get(field) for field in FIELD_NAMES])