docs: def _create_index_formula_lookup(formula_id2index,
                                   feature_folder,
                                   index2latex):
      """
      Create a lookup file where the index is mapped to the formula id and the
      LaTeX command.
  
      Parameters
      ----------
      formula_id2index : dict
      feature_folder : str
          Path to a folder in which a feature file as well as an
          index2formula_id.csv is.
      index2latex : dict
          Maps an integer index to a LaTeX command
      """
      index2formula_id = sorted(formula_id2index.items(), key=lambda n: n[1])
      index2formula_file = os.path.join(feature_folder, "index2formula_id.csv")
      with open(index2formula_file, "w") as f:
          f.write("index,formula_id,latex\n")
          for formula_id, index in index2formula_id:
              f.write("%i,%i,%s\n" % (index, formula_id, index2latex[index]))