src/transformers/models/marian/convert_marian_to_pytorch.py/add_to_vocab_
def add_to_vocab_(vocab: Dict[str, int], special_tokens: List[str]):
    start = max(vocab.values()) + 1
    added = 0
    for tok in special_tokens:
        if tok in vocab:
            continue
        vocab[tok] = start + added
        added += 1
    return added