def truncate_segments(tokens_a, tokens_b, len_a, len_b, max_num_tokens, np_rng):
    """Truncates a pair of sequences to a maximum sequence length."""
    # print(len_a, len_b, max_num_tokens)
    assert len_a > 0
    if len_a + len_b <= max_num_tokens:
        return False
    while len_a + len_b > max_num_tokens:
        if len_a > len_b:
            len_a -= 1
            tokens = tokens_a
        else:
            len_b -= 1
            tokens = tokens_b
        if np_rng.random() < 0.5:
            del tokens[0]
        else:
            tokens.pop()
    return True