    def append(self, other_dataset_path_prefix: str) -> None:
        """
        append an existing dataset.
        Beware, if it wasn't built with the same coder, you are in trouble.
        """
        other_index = HuffmanMMapIndex(
            indexed_dataset.index_file_path(other_dataset_path_prefix)
        )
        for (ptr, size) in other_index:
            self._ptrs.append(ptr + self._data_len)
            self._sizes.append(size)

        # Concatenate data
        with open(indexed_dataset.data_file_path(other_dataset_path_prefix), "rb") as f:
            shutil.copyfileobj(f, self._data_file)

        self._data_len += other_index.data_len
append an existing dataset.
Beware, if it wasn't built with the same coder, you are in trouble.
fairseq/data/huffman/huffman_mmap_indexed_dataset.py