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

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]))