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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _tokens_to_subtoken(self, tokens):
 """ Converts a list of tokens to a list of subtoken. Args: tokens: a list of strings. Returns: a list of integers in the range [0, vocab_size) """

        ret = []
        for token in tokens:
            ret.extend(
                self._escaped_token_to_subtoken_strings(_escape_token(token, self._alphabet)))
        return ret