Combinartion of priorfactors from all populations

def priorfactors(self):
        """Combinartion of priorfactors from all populations
        """
        priorfactors = {}
        for pop in self.poplist:
            for f in pop.priorfactors:
                if f in priorfactors:
                    if pop.priorfactors[f] != priorfactors[f]:
                        raise ValueError('prior factor %s is inconsistent!' % f)
                else:
                    priorfactors[f] = pop.priorfactors[f]
        return priorfactors