code: def mac(self, mac):
          
          self._mac = mac
          if mac:
              self.duid = ib_utils.generate_duid(mac)
          elif not hasattr(self, 'duid'):
              self.duid = None
docs: Set mac and duid fields
  
          To have common interface with FixedAddress accept mac address
          and set duid as a side effect.
          'mac' was added to _shadow_fields to prevent sending it out over wapi.