msub-09.test

<<< native
[ ESub
    (EIdentifier "A")
    (ESub (EIdentifier "B") (ESub (EIdentifier "C") (EIdentifier "D")))
, ESymbol Rel "="
, ESub
    (EIdentifier "A")
    (ESub
       (EIdentifier "B")
       (ESub
          (EIdentifier "C")
          (ESub (EIdentifier "D") (EPhantom (EIdentifier "E")))))
]
>>> mml
<?xml version='1.0' ?>
<math display="block" xmlns="http://www.w3.org/1998/Math/MathML">
  <mrow>
    <msub>
      <mi>A</mi>
      <msub>
        <mi>B</mi>
        <msub>
          <mi>C</mi>
          <mi>D</mi>
        </msub>
      </msub>
    </msub>
    <mo>=</mo>
    <msub>
      <mi>A</mi>
      <msub>
        <mi>B</mi>
        <msub>
          <mi>C</mi>
          <msub>
            <mi>D</mi>
            <mphantom>
              <mi>E</mi>
            </mphantom>
          </msub>
        </msub>
      </msub>
    </msub>
  </mrow>
</math>