docs: def _load_vocab_file(vocab_file, reserved_tokens=None):
    """Load vocabulary while ensuring reserved tokens are at the top."""
    if reserved_tokens is None:
      reserved_tokens = RESERVED_TOKENS
  
    subtoken_list = []
    with tf.gfile.Open(vocab_file, mode="r") as f:
      for line in f:
        subtoken = _native_to_unicode(line.strip())
        subtoken = subtoken[1:-1]  # Remove surrounding single-quotes
        if subtoken in reserved_tokens:
          continue
        subtoken_list.append(_native_to_unicode(subtoken))
    return reserved_tokens + subtoken_list