msubsup-10.test

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

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">  <msubsup>    <mi>p</mi>     <mi>x</mi>     <mn>3</mn>   </msubsup>   <mo>=</mo>   <munder>    <mrow>      <msubsup>        <mi>p</mi>         <mi>x</mi>         <mn>3</mn>       </msubsup>     </mrow>     <mphantom>      <mtext>l</mtext>
    </mphantom>   </munder> </math>
>>> native
[ ESubsup (EIdentifier "p") (EIdentifier "x") (ENumber "3")
, ESymbol Rel "="
, EUnder
    False
    (ESubsup (EIdentifier "p") (EIdentifier "x") (ENumber "3"))
    (EPhantom (EText TextNormal "l"))
]