<<< native
[ EUnder
    False
    (EIdentifier "BASE")
    (EFraction
       NormalFrac
       (EIdentifier "first under script")
       (EIdentifier "second one"))
, ESymbol Rel "="
, EUnder
    False
    (EUnder
       False
       (EIdentifier "BASE")
       (EFraction
          NormalFrac
          (EIdentifier "first under script")
          (EIdentifier "second one")))
    (EPhantom (EIdentifier "fail"))
]
>>> mml
<?xml version='1.0' ?>
<math display="block" xmlns="http://www.w3.org/1998/Math/MathML">
  <mrow>
    <munder>
      <mi>BASE</mi>
      <mfrac>
        <mi>first under script</mi>
        <mi>second one</mi>
      </mfrac>
    </munder>
    <mo>=</mo>
    <munder>
      <munder>
        <mi>BASE</mi>
        <mfrac>
          <mi>first under script</mi>
          <mi>second one</mi>
        </mfrac>
      </munder>
      <mphantom>
        <mi>fail</mi>
      </mphantom>
    </munder>
  </mrow>
</math>