<<< native
[ EUnder
    True
    (ESymbol Op "\8721")
    (EArray
       [ AlignCenter ]
       [ [ [ EGrouped
               [ ENumber "0"
               , ESymbol Rel "<"
               , EIdentifier "i"
               , ESymbol Rel "<"
               , EIdentifier "m"
               ]
           ]
         ]
       , [ [ EGrouped
               [ ENumber "0"
               , ESymbol Rel "<"
               , EIdentifier "j"
               , ESymbol Rel "<"
               , EIdentifier "n"
               ]
           ]
         ]
       ])
, EIdentifier "P"
, EDelimited
    "("
    ")"
    [ Right (EIdentifier "i")
    , Right (ESymbol Pun ",")
    , Right (EIdentifier "j")
    ]
]
>>> omml
<?xml version='1.0' ?>
<m:oMathPara>
  <m:oMathParaPr>
    <m:jc m:val="center" />
  </m:oMathParaPr>
  <m:oMath>
    <m:nary>
      <m:naryPr>
        <m:chr m:val="∑" />
        <m:limLoc m:val="undOvr" />
        <m:subHide m:val="0" />
        <m:supHide m:val="1" />
      </m:naryPr>
      <m:sub>
        <m:m>
          <m:mPr>
            <m:baseJc m:val="center" />
            <m:plcHide m:val="1" />
            <m:mcs>
              <m:mc>
                <m:mcPr>
                  <m:mcJc m:val="center" />
                  <m:count m:val="1" />
                </m:mcPr>
              </m:mc>
            </m:mcs>
          </m:mPr>
          <m:mr>
            <m:e>
              <m:r>
                <m:t>0</m:t>
              </m:r>
              <m:r>
                <m:rPr>
                  <m:sty m:val="p" />
                </m:rPr>
                <m:t>&lt;</m:t>
              </m:r>
              <m:r>
                <m:t>i</m:t>
              </m:r>
              <m:r>
                <m:rPr>
                  <m:sty m:val="p" />
                </m:rPr>
                <m:t>&lt;</m:t>
              </m:r>
              <m:r>
                <m:t>m</m:t>
              </m:r>
            </m:e>
          </m:mr>
          <m:mr>
            <m:e>
              <m:r>
                <m:t>0</m:t>
              </m:r>
              <m:r>
                <m:rPr>
                  <m:sty m:val="p" />
                </m:rPr>
                <m:t>&lt;</m:t>
              </m:r>
              <m:r>
                <m:t>j</m:t>
              </m:r>
              <m:r>
                <m:rPr>
                  <m:sty m:val="p" />
                </m:rPr>
                <m:t>&lt;</m:t>
              </m:r>
              <m:r>
                <m:t>n</m:t>
              </m:r>
            </m:e>
          </m:mr>
        </m:m>
      </m:sub>
      <m:sup>
        <m:r>
          <m:t>&#8203;</m:t>
        </m:r>
      </m:sup>
      <m:e>
        <m:r>
          <m:t>P</m:t>
        </m:r>
      </m:e>
    </m:nary>
    <m:d>
      <m:dPr>
        <m:begChr m:val="(" />
        <m:endChr m:val=")" />
        <m:sepChr m:val="" />
        <m:grow />
      </m:dPr>
      <m:e>
        <m:r>
          <m:t>i</m:t>
        </m:r>
        <m:r>
          <m:rPr>
            <m:sty m:val="p" />
          </m:rPr>
          <m:t>,</m:t>
        </m:r>
        <m:r>
          <m:t>j</m:t>
        </m:r>
      </m:e>
    </m:d>
  </m:oMath>
</m:oMathPara>