Basic test of <mphantom> with an inferred <mrow>.
<math xmlns="http://www.w3.org/1998/Math/MathML">
<mrow>
<mi>A</mi>
<mphantom>
<mi>A</mi>
</mphantom>
<mi>A</mi>
<mphantom>
<mi>A</mi>
<mo>+</mo>
<mi>A</mi>
</mphantom>
<mi>A</mi>
</mrow>
</math>
Author: Robert Miner and Jeff Schaefer, Geometry Technologies