lue)

     case(
         x >= 0 : x,
         Negative-Number!
     )
 }

 /attribute(valid-int, target-node, valid-int)

 # Initial Values

 0 -> a
 0 -> b
?>
<!doctype html>
<html>
    <head>
        <title>Adding Numbers</title>
    </head>
    <body>
      <h1>Adding Numbers</h1>
      <div>
        <label>A: <input id="a" value="<?@ valid-int(a) ?>"/></label>
        <?@ error-message(a) ?>
      </div>
      <div>
        <label>B: <input id="b" value="<?@ valid-int(b) ?>"/></label>
        <?@ error