<<< mml
<?xml version="1.0" encoding="UTF-8"?>

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">  <msup>    <mi>c</mi>     <mn>4</mn>   </msup>   <mo>=</mo>   <msup>    <mi>c</mi>     <mrow>      <mn>4</mn>
      <mphantom>
        <mi>fail</mi>
      </mphantom>
    </mrow>   </msup> </math>
>>> native
[ ESuper (EIdentifier "c") (ENumber "4")
, ESymbol Rel "="
, ESuper
    (EIdentifier "c")
    (EGrouped [ ENumber "4" , EPhantom (EIdentifier "fail") ])
]