<<< native
[ EIdentifier "a"
, ESuper (EIdentifier "x") (ENumber "2")
, ESymbol Bin "+"
, EIdentifier "b"
, EIdentifier "x"
, ESymbol Bin "+"
, EIdentifier "c"
, ESymbol Rel "="
, ENumber "0"
]
>>> mml
<?xml version='1.0' ?>
<math display="block" xmlns="http://www.w3.org/1998/Math/MathML">
  <mrow>
    <mi>a</mi>
    <msup>
      <mi>x</mi>
      <mn>2</mn>
    </msup>
    <mo>+</mo>
    <mi>b</mi>
    <mi>x</mi>
    <mo>+</mo>
    <mi>c</mi>
    <mo>=</mo>
    <mn>0</mn>
  </mrow>
</math>