<<< native
[ EDelimited
    "("
    ")"
    [ Right
        (EGrouped
           [ ESub (EIdentifier "a") (ENumber "1")
           , ESymbol Pun ","
           , ESub (EIdentifier "a") (ENumber "2")
           , ESymbol Pun ","
           , ESub (EIdentifier "a") (ENumber "3")
           ])
    ]
]
>>> omml
<?xml version='1.0' ?>
<m:oMathPara>
  <m:oMathParaPr>
    <m:jc m:val="center" />
  </m:oMathParaPr>
  <m:oMath>
    <m:d>
      <m:dPr>
        <m:begChr m:val="(" />
        <m:endChr m:val=")" />
        <m:sepChr m:val="" />
        <m:grow />
      </m:dPr>
      <m:e>
        <m:sSub>
          <m:e>
            <m:r>
              <m:t>a</m:t>
            </m:r>
          </m:e>
          <m:sub>
            <m:r>
              <m:t>1</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>a</m:t>
            </m:r>
          </m:e>
          <m:sub>
            <m:r>
              <m:t>2</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>a</m:t>
            </m:r>
          </m:e>
          <m:sub>
            <m:r>
              <m:t>3</m:t>
            </m:r>
          </m:sub>
        </m:sSub>
      </m:e>
    </m:d>
  </m:oMath>
</m:oMathPara>