support linebreaks in munder
It would be nice if something like the following would work.
<math xmlns="http://www.w3.org/1998/Math/MathML">
<munder>
<mrow class="MJX-TeXAtom-OP MJX-fixedlimits">
<munder>
<mrow>
<mi>f</mi>
<mo stretchy="false">(</mo>
<mi>t</mi>
<mo>+</mo>
<mn>1</mn>
<mo stretchy="false">)</mo>
</mrow>
<mo>⏟<!-- ⏟ --></mo>
</munder>
</mrow>
<mrow class="MJX-TeXAtom-ORD">
<mtext>distance at </mtext>
<mo linebreak="newline"></mo>
<mtext>time </mtext>
<mi>t</mi>
<mo>+</mo>
<mn>1</mn>
</mrow>
</munder>
</math>