def is_hidden(self):
        return self.related_name and self.related_name[-1] == '+'
Ensure we disallow negative admin fee or attendance.
dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-1.5/django/db/models/fields/related.py/ManyToManyRel.is_hidden--crossover with--dataset/ETHPy150Open swcarpentry/amy/workshops/test/test_event.py/TestEventViews.test_negative_admin_fee_attendance