<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
<mml:mfenced close="|" open="|"> <mml:mover accent="true"> <mml:mi>M</mml:mi>
<mml:mo stretchy="true">^</mml:mo>
</mml:mover>
</mml:mfenced>
</mml:math>