<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def convert_subject_ids(self, subject_ids):
 """ Convert subject ids to strings if they are integers """

        # TODO: need to make this generalisable via a
        #       splitting+mapping function passed to the repository
        if subject_ids is not None:
            subject_ids = set(
                ('{:03d}'.format(s)
                 if isinstance(s, int) else s) for s in subject_ids)
        return subject_ids