docs: 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