<?xml version="1.0" encoding="UTF-8"?>

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">  <munder>    <mi mathvariant="normal">B</mi>     <mi mathvariant="normal">long under script</mi>   </munder>   <mo>=</mo>   <munder>    <mrow>      <mphantom>        <mtext>x</mtext>
      </mphantom>
      <mi mathvariant="normal">B</mi>
      <mphantom>
        <mtext>x</mtext>
      </mphantom>
    </mrow>     <mrow>      <mi mathvariant="normal">long under script</mi>
    </mrow>   </munder> </math>