`ul` (unary)

Ul

AsciiMath

ul(x)

LaTeX

\underline{x}

MathML

<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
  <mstyle displaystyle="true">
    <munder>
      <mi>x</mi>
      <mo>&#x332;</mo>
    </munder>
  </mstyle>
</math>

OMML

<m:oMathPara>
  <m:oMath>
    <m:limLow>
      <m:limLowPr>
        <m:ctrlPr>
          <w:rPr>
            <w:rFonts w:ascii="Cambria Math" w:hAnsi="Cambria Math"/>
            <w:i/>
          </w:rPr>
        </m:ctrlPr>
      </m:limLowPr>
      <m:e>
        <m:r>
          <m:t>x</m:t>
        </m:r>
      </m:e>
      <m:lim>
        <m:r>
          <m:t>&#x332;</m:t>
        </m:r>
      </m:lim>
    </m:limLow>
  </m:oMath>
</m:oMathPara>