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.

def preprocess(self, xs):
        """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.
        """
        return [self.nesting_field.preprocess(x)
                for x in super(NestedField, self).preprocess(xs)]