docs: def segment(self, *args):
          """Segment one or more datasets with this subword field.
  
          Arguments:
              Positional arguments: Dataset objects or other indexable
                  mutable sequences to segment. If a Dataset object is provided,
                  all columns corresponding to this field are used; individual
                  columns can also be provided directly.
          """
          sources = []
          for arg in args:
              if isinstance(arg, Dataset):
                  sources += [getattr(arg, name) for name, field in
                              arg.fields.items() if field is self]
              else:
                  sources.append(arg)
          for data in sources:
              for x in tqdm(data, 'segmenting'):
                  x[:] = self.vocab.segment(x)