def get_prev_step(self, step=None):
        if step is None:
            step = self.steps.current
        form_list = self.get_form_list()
        key = form_list.keyOrder.index(step) - 1
        if key >= 0:
            return form_list.keyOrder[key]
        return None
Add a new method to this interface and return the method.
dataset/ETHPy150Open AppScale/appscale/AppServer/lib/django-1.5/django/contrib/formtools/wizard/views.py/WizardView.get_prev_step--crossover with--dataset/ETHPy150Open pdef/pdef/compiler/pdefc/lang.py/Interface.create_method