while 1
   resp = input('Low (0), high (1), undo(2), exit(3): ');
   if length(resp) == 0
      continue;
   end
   switch resp
   case 0
      'low'
   case 1
      'high'
   case 2
      'undo'
   case 3
      break;
   otherwise
      continue;
   end
end
