<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _list_patient_ids(self):
 """ Utility function to return a list of patient ids in the Cohort """

        results = []
        for patient in self:
            results.append(patient.id)
        return(results)