<math xmlns="http://www.w3.org/1998/Math/MathML">
<mi>x</mi>
<mo>+</mo>
<munder>
<mpadded depth="-10px">
<mi>x</mi>
<mo>+</mo>
<mi>y</mi>
</mpadded>
<mo>̲</mo>
</munder>
<mo>+</mo>
<munder>
<mrow>
<mi>x</mi>
<mo>+</mo>
<mi>y</mi>
</mrow>
<mo>̲</mo>
</munder>
</math>
Author: Design Science, Inc. (D. Doyle, E. Cannon, E. Tabacman, R.Miner)