[monolithed]

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    document.forma.onsubmit = function(){
        this.elements[1].value = eval(this.elements[0].value);
        return false;
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<form action="" name="forma" method="post">
<div id=pl>
    <p>Ваш пример:</p>
    <input type="text" value="" />
    <p>Результат:</p>    
    <input type="text" value="" />
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</form>