msub-03.test

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

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">  <msub>    <mi>V</mi>     <mo>+</mo>   </msub>   <mo>=</mo>   <msub>    <mi>V</mi>     <mrow>      <msub>        <mo>+</mo>         <mphantom>          <mi mathvariant="normal">fail</mi>
        </mphantom>       </msub>     </mrow>   </msub> </math>
>>> native
[ ESub (EIdentifier "V") (ESymbol Bin "+")
, ESymbol Rel "="
, ESub
    (EIdentifier "V")
    (ESub (ESymbol Bin "+") (EPhantom (EIdentifier "fail")))
]