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<form id="calculatorForm">
  <fieldset>
    <input type="checkbox" id="enableCalc">
    <label for="enableCalc">Enable calc</label>
  </fieldset>
  
  <fieldset name="calculator" disabled>
    <!-- Default formula -->
    <fieldset name="defaultcalc">
      <input type="checkbox" id="default" checked>
      <label for="default">Use default calculation</label>
    </fieldset>
    
    <!-- Custom formula -->
    <fieldset name="customcalc" disabled>
      <label>Use custom calculation</label>
      <input type="number" value="50" id="c">+
      <input type="number" value="50" id="d">=
      <output id="x" for="c d">100</output>
    </fieldset>
  </fieldset>
</form>