msup-12.test

<<< native
[ ESuper (EIdentifier "a") (ESymbol Bin "+")
, ESymbol Rel "="
, EUnderover
    False
    (ESuper (EIdentifier "a") (ESymbol Bin "+"))
    (EPhantom (EIdentifier "x"))
    (EPhantom (EIdentifier "x"))
]
>>> mml
<?xml version='1.0' ?>
<math display="block" xmlns="http://www.w3.org/1998/Math/MathML">
  <mrow>
    <msup>
      <mi>a</mi>
      <mo>+</mo>
    </msup>
    <mo>=</mo>
    <munderover>
      <msup>
        <mi>a</mi>
        <mo>+</mo>
      </msup>
      <mphantom>
        <mi>x</mi>
      </mphantom>
      <mphantom>
        <mi>x</mi>
      </mphantom>
    </munderover>
  </mrow>
</math>