<?xml version="1.0"?>
<!DOCTYPE math PUBLIC "-//W3C//DTD MathML 2.0//EN" "http://www.w3.org/TR/MathML2/dtd/mathml2.dtd">
    <math mode="display">
      <msubsup>
        <mi>x</mi>
        <msubsup>
          <mi>y</mi>
          <mi>b</mi>
          <mi>a</mi>
        </msubsup>
        <msubsup>
          <mi>z</mi>
          <mi>c</mi>
          <mi>d</mi>
        </msubsup>
      </msubsup>
    </math>
