src/transformers/models/deberta_v2/tokenization_deberta_v2.py/SPMTokenizer/add_special_token
class SPMTokenizer:    def add_special_token(self, token):
        if token not in self.special_tokens:
            self.special_tokens.append(token)
            if token not in self.vocab:
                self.vocab[token] = len(self.vocab) - 1
                self.ids_to_tokens.append(token)
        return self.id(token)