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

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">  <mover>    <mi mathvariant="normal">_ baseline _</mi>     <mi mathvariant="normal">over script</mi>   </mover>   <mo>=</mo>   <mover>    <mrow>      <munder>        <mi mathvariant="normal">_ baseline _</mi>         <mphantom>          <mi mathvariant="normal">fail</mi>
        </mphantom>       </munder>     </mrow>     <mi mathvariant="normal">over script</mi>   </mover> </math>
>>> native
[ EOver
    False (EIdentifier "_ baseline _") (EIdentifier "over script")
, ESymbol Rel "="
, EOver
    False
    (EUnder
       False (EIdentifier "_ baseline _") (EPhantom (EIdentifier "fail")))
    (EIdentifier "over script")
]