code: protected function isNoResultMask($mask) {
      foreach ($this->noResultMasks as $noresultMask) {
        if ($mask === $noresultMask) {
          return TRUE; // @codeCoverageIgnore
        }
        if (($mask & $noresultMask) === $noresultMask) {
          $this->noResultMasks[] = $mask;
          return TRUE;
        }
      }
      return FALSE;
    }
docs: Try to determine if this mask will result in no intersections based on
  previously generated masks.
  
  @param string $mask
  The mask to check.
  
  @return boolean
  TRUE if this mask would result in an empty intersection, FALSE if not or
  if not known.