<<< native
[ EUnder
    False
    (EIdentifier "BASE")
    (EFraction
       NormalFrac
       (EIdentifier "first under script")
       (EIdentifier "second one"))
, ESymbol Rel "="
, EUnder
    False
    (EUnder
       False
       (EIdentifier "BASE")
       (EFraction
          NormalFrac
          (EIdentifier "first under script")
          (EIdentifier "second one")))
    (EPhantom (EIdentifier "fail"))
]
>>> omml
<?xml version='1.0' ?>
<m:oMathPara>
  <m:oMathParaPr>
    <m:jc m:val="center" />
  </m:oMathParaPr>
  <m:oMath>
    <m:limLow>
      <m:e>
        <m:r>
          <m:t>BASE</m:t>
        </m:r>
      </m:e>
      <m:lim>
        <m:f>
          <m:fPr>
            <m:type m:val="bar" />
          </m:fPr>
          <m:num>
            <m:r>
              <m:t>first under script</m:t>
            </m:r>
          </m:num>
          <m:den>
            <m:r>
              <m:t>second one</m:t>
            </m:r>
          </m:den>
        </m:f>
      </m:lim>
    </m:limLow>
    <m:r>
      <m:rPr>
        <m:sty m:val="p" />
      </m:rPr>
      <m:t>=</m:t>
    </m:r>
    <m:limLow>
      <m:e>
        <m:limLow>
          <m:e>
            <m:r>
              <m:t>BASE</m:t>
            </m:r>
          </m:e>
          <m:lim>
            <m:f>
              <m:fPr>
                <m:type m:val="bar" />
              </m:fPr>
              <m:num>
                <m:r>
                  <m:t>first under script</m:t>
                </m:r>
              </m:num>
              <m:den>
                <m:r>
                  <m:t>second one</m:t>
                </m:r>
              </m:den>
            </m:f>
          </m:lim>
        </m:limLow>
      </m:e>
      <m:lim>
        <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:lim>
    </m:limLow>
  </m:oMath>
</m:oMathPara>