<<< native
[ EArray
    [ AlignCenter ]
    [ [ [ EArray [ AlignCenter ] [ [ [ EIdentifier "x" ] ] ] ] ] ]
]
>>> mml
<?xml version='1.0' ?>
<math display="block" xmlns="http://www.w3.org/1998/Math/MathML">
  <mtable>
    <mtr>
      <mtd columnalign="center" style="text-align: center">
        <mtable>
          <mtr>
            <mtd columnalign="center" style="text-align: center">
              <mi>x</mi>
            </mtd>
          </mtr>
        </mtable>
      </mtd>
    </mtr>
  </mtable>
</math>