msub-01.test

<<< native
[ ESub (EIdentifier "L") (EIdentifier "x")
, ESymbol Rel "="
, ESub
    (EIdentifier "L")
    (EGrouped [ EIdentifier "x" , EPhantom (EIdentifier "fail") ])
]
>>> omml
<?xml version='1.0' ?>
<m:oMathPara>
  <m:oMathParaPr>
    <m:jc m:val="center" />
  </m:oMathParaPr>
  <m:oMath>
    <m:sSub>
      <m:e>
        <m:r>
          <m:t>L</m:t>
        </m:r>
      </m:e>
      <m:sub>
        <m:r>
          <m:t>x</m:t>
        </m:r>
      </m:sub>
    </m:sSub>
    <m:r>
      <m:rPr>
        <m:sty m:val="p" />
      </m:rPr>
      <m:t>=</m:t>
    </m:r>
    <m:sSub>
      <m:e>
        <m:r>
          <m:t>L</m:t>
        </m:r>
      </m:e>
      <m:sub>
        <m:r>
          <m:t>x</m:t>
        </m:r>
        <m:phant>
          <m:phantPr>
            <m:show m:val="0" />
          </m:phantPr>
          <m:e>
            <m:r>
              <m:t>fail</m:t>
            </m:r>
          </m:e>
        </m:phant>
      </m:sub>
    </m:sSub>
  </m:oMath>
</m:oMathPara>