<?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>