docs: def _generate_substrings(self, ngram, size):
          """Returns a list of all substrings of `ngram`.
  
          :param ngram: n-gram to generate substrings of
          :type ngram: `str`
          :param size: size of `ngram`
          :type size: `int`
          :rtype: `list`
  
          """
          text = Text(ngram, self._tokenizer)
          substrings = []
          for sub_size, ngrams in text.get_ngrams(1, size-1):
              for sub_ngram, count in ngrams.items():
                  substrings.extend([sub_ngram] * count)
          return substrings