code: def preprocess(self, xs):
          
          return [self.nesting_field.preprocess(x)
                  for x in super(NestedField, self).preprocess(xs)]
docs: Preprocess a single example.
  
          Firstly, tokenization and the supplied preprocessing pipeline is applied. Since
          this field is always sequential, the result is a list. Then, each element of
          the list is preprocessed using ``self.nesting_field.preprocess`` and the resulting
          list is returned.
  
          Arguments:
              xs (list or str): The input to preprocess.
  
          Returns:
              list: The preprocessed list.