def _clean_bow(self) -> None:
        """Remove words that do not exceed `self.delete_min_df`."""
        # Only keep words with a minimum frequency
        indices = np.where(self.X_.sum(0) >= self.delete_min_df)[1]
        indices_dict = {index: index for index in indices}
        self.X_ = self.X_[:, indices]

        # Update vocabulary with new words
        new_vocab = {}
        vocabulary_dict = {v: k for k, v in self.vocabulary_.items()}
        for i, index in enumerate(indices):
            if indices_dict.get(index) is not None:
                new_vocab[vocabulary_dict[index]] = i

        self.vocabulary_ = new_vocab
Remove words that do not exceed `self.delete_min_df`.
bertopic/vectorizers/_online_cv.py