code: def _add_to_checksum(self, checksum, value):
          
          checksum = self._byte_rot_left(checksum, 1)
          checksum = checksum + value
          if (checksum > 255):
              checksum = checksum - 255
          self._debug(PROP_LOGLEVEL_TRACE, "C: " + str(checksum) + " V: " + str(value))
          return checksum
docs: Add a byte to the checksum.