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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 to_indices(self, tokens):
 """Converts tokens to indices according to the vocabulary. Parameters tokens : str or list of strs A source token or tokens to be converted. Returns ------- int or list of ints A token index or a list of token indices according to the vocabulary. """


        to_reduce = False
        if not isinstance(tokens, list):
            tokens = [tokens]
            to_reduce = True

        indices = [self.token_to_idx[token] if token in self.token_to_idx
                   else C.UNKNOWN_IDX for token in tokens]

        return indices[0] if to_reduce else indices