<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _fill_untouched(idx, ret, fill_value):
 """any elements of ret not indexed by idx are set to fill_value."""

    untouched = np.ones_like(ret, dtype=bool)
    untouched[idx] = False
    ret[untouched] = fill_value