code: def to_indices(self, tokens):
          
  
          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
docs: 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.