def create_tokens_and_tokentypes(tokens_a, tokens_b, cls_id, sep_id):
    """Merge segments A and B, add [CLS] and [SEP] and build tokentypes."""

    tokens = []
    tokentypes = []
    # [CLS].
    tokens.append(cls_id)
    tokentypes.append(0)
    # Segment A.
    for token in tokens_a:
        tokens.append(token)
        tokentypes.append(0)
    # [SEP].
    tokens.append(sep_id)
    tokentypes.append(0)
    # Segment B.
    for token in tokens_b:
        tokens.append(token)
        tokentypes.append(1)
    if tokens_b:
        # [SEP].
        tokens.append(sep_id)
        tokentypes.append(1)

    return tokens, tokentypes