code: def save_to_file(self, filename_prefix):
      
      # Wrap in single quotes to make it easier to see the full subword when
      # it has spaces and make it easier to search with ctrl+f.
      filename = self._filename(filename_prefix)
      lines = ["'%s'" % s for s in self._subwords]
      self._write_lines_to_file(filename, lines)
docs: Save the vocabulary to a file.