docs: def filter_examples(self, field_names):
          """Remove unknown words from dataset examples with respect to given field.
  
          Arguments:
              field_names (list(str)): Within example only the parts with field names in
                  field_names will have their unknown words deleted.
          """
          for i, example in enumerate(self.examples):
              for field_name in field_names:
                  vocab = set(self.fields[field_name].vocab.stoi)
                  text = getattr(example, field_name)
                  example_part = [word for word in text if word in vocab]
                  setattr(example, field_name, example_part)
              self.examples[i] = example