How to use the reprozip.utils.flatten function in reprozip

files_set = set(in_files) | set(out_files)
        nb_files = 0
        for arg_nb, arg in enumerate(run['argv']):
            p = Path(run['workingdir'], arg).resolve()
            if p in files_set:
                nb_files += 1
                if p not in runs_with_file:
                    runs_with_file[p] = run_nb, arg_nb
                elif runs_with_file[p] is not None:
                    runs_with_file[p] = None
        nb_file_args.append(nb_files)

    file_names = {}
    make_unique = UniqueNames()

    for fi in flatten(2, (inputs, outputs)):
        if fi in file_names:
            continue

        # If it appears in at least one of the command-lines
        if fi in runs_with_file:
            # If it only appears once in the command-lines
            if runs_with_file[fi] is not None:
                run_nb, arg_nb = runs_with_file[fi]
                parts = []
                # Run number, if there are more than one runs
                if len(runs) > 1:
                    parts.append(run_nb)
                # Argument number, if there are more than one file arguments
                if nb_file_args[run_nb] > 1:
                    parts.append(arg_nb)
                file_names[fi] = make_unique(

files_set = set(in_files) | set(out_files)
        nb_files = 0
        for arg_nb, arg in enumerate(run['argv']):
            p = Path(run['workingdir'], arg).resolve()
            if p in files_set:
                nb_files += 1
                if p not in runs_with_file:
                    runs_with_file[p] = run_nb, arg_nb
                elif runs_with_file[p] is not None:
                    runs_with_file[p] = None
        nb_file_args.append(nb_files)

    file_names = {}
    make_unique = UniqueNames()

    for fi in flatten(2, (inputs, outputs)):
        if fi in file_names:
            continue

        # If it appears in at least one of the command-lines
        if fi in runs_with_file:
            # If it only appears once in the command-lines
            if runs_with_file[fi] is not None:
                run_nb, arg_nb = runs_with_file[fi]
                parts = []
                # Run number, if there are more than one runs
                if len(runs) > 1:
                    parts.append(run_nb)
                # Argument number, if there are more than one file arguments
                if nb_file_args[run_nb] > 1:
                    parts.append(arg_nb)
                file_names[fi] = make_unique(