msup-10.test

<<< native
[ ESuper (EIdentifier "S") (ENumber "2")
, ESymbol Rel "="
, EUnder
    False
    (ESuper (EIdentifier "S") (ENumber "2"))
    (EPhantom (EIdentifier "fail"))
]
>>> mml
<?xml version='1.0' ?>
<math display="block" xmlns="http://www.w3.org/1998/Math/MathML">
  <mrow>
    <msup>
      <mi>S</mi>
      <mn>2</mn>
    </msup>
    <mo>=</mo>
    <munder>
      <msup>
        <mi>S</mi>
        <mn>2</mn>
      </msup>
      <mphantom>
        <mi>fail</mi>
      </mphantom>
    </munder>
  </mrow>
</math>