docs: def sort_by_formula_id(raw_datasets):
      """
      Sort a list of formulas by `id`, where `id` represents the accepted
      formula id.
  
      Parameters
      ----------
      raw_datasets : list of dictionaries
          A list of raw datasets.
  
      Examples
      --------
      The parameter `raw_datasets` has to be of the format
  
      >>> rd = [{'is_in_testset': 0,
      ...        'formula_id': 31,
      ...        'handwriting': HandwrittenData(raw_data_id=2953),
      ...        'formula_in_latex': 'A',
      ...        'id': 2953},
      ...       {'is_in_testset': 0,
      ...        'formula_id': 31,
      ...        'handwriting': HandwrittenData(raw_data_id=4037),
      ...        'formula_in_latex': 'A',
      ...        'id': 4037},
      ...       {'is_in_testset': 0,
      ...        'formula_id': 31,
      ...        'handwriting': HandwrittenData(raw_data_id=4056),
      ...        'formula_in_latex': 'A',
      ...        'id': 4056}]
      >>> sort_by_formula_id(rd)
      """
      by_formula_id = defaultdict(list)
      for el in raw_datasets:
          by_formula_id[el['handwriting'].formula_id].append(el['handwriting'])
      return by_formula_id