sub example {
      do {
          $av = irand(11) + 10;
          $bv = irand(11);
          $cv = irand(2) ? "+" : "-";
      } until ( !( $av - $bv < 0 || $av + $bv >= 20 ) );
      print $av. $cv . $bv . " = \n";
  }