$use StdIO Class Box;

$box B;

Main =
  {
    <Box? <Box 34>>;
    <PrintLN 'fail'>;
  },
  {
    <Box? &B>;
    <PrintLN 'fail'>;
  },
  {
    <Box? 23>;
    <PrintLN 'fail: argument is not a box'>;
  },
  {
    <Box? <Box 34> <Box 34>>;
    <PrintLN 'fail: argument is more than one expression'>;
  };