substack.test

<<< 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")
    ]
]
>>> mml
<?xml version='1.0' ?>
<math display="block" xmlns="http://www.w3.org/1998/Math/MathML">
  <mrow>
    <munder>
      <mo>∑</mo>
      <mtable>
        <mtr>
          <mtd columnalign="center">
            <mrow>
              <mn>0</mn>
              <mo>&lt;</mo>
              <mi>i</mi>
              <mo>&lt;</mo>
              <mi>m</mi>
            </mrow>
          </mtd>
        </mtr>
        <mtr>
          <mtd columnalign="center">
            <mrow>
              <mn>0</mn>
              <mo>&lt;</mo>
              <mi>j</mi>
              <mo>&lt;</mo>
              <mi>n</mi>
            </mrow>
          </mtd>
        </mtr>
      </mtable>
    </munder>
    <mi>P</mi>
    <mrow>
      <mo stretchy="true" form="prefix">(</mo>
      <mi>i</mi>
      <mo>,</mo>
      <mi>j</mi>
      <mo stretchy="true" form="postfix">)</mo>
    </mrow>
  </mrow>
</math>