<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _write_subset_index_file(options, core_results):
 """ Write table giving index of subsets, giving number and subset string """


    f_path = os.path.join(options['run_dir'], '_subset_index.csv')
    subset_strs = zip(*core_results)[0]
    index = np.arange(len(subset_strs)) + 1
    df = pd.DataFrame({'subsets': subset_strs}, index=index)
    df.to_csv(f_path)