code: def constrain_oddeven(self, diff, **kwargs):
          
          if 'odd-even' not in self.constraints:
              self.constraints.append('odd-even')
          for pop in self.poplist:
              if isinstance(pop, EclipsePopulation_Px2):
                  pop.constrain_oddeven(diff, **kwargs)
          self.oddeven_diff = diff
docs: Constrains the difference b/w primary and secondary to be < diff