<<< native
[ EArray
    [ AlignCenter , AlignCenter , AlignCenter ]
    [ [ [ EIdentifier "H" ]
      , [ ESymbol Accent "\8594" ]
      , [ EIdentifier "G" ]
      ]
    , [ [ ESymbol Rel "\8595" ]
      , [ ESpace (3 % 1) ]
      , [ ESymbol Rel "\8595" ]
      ]
    , [ [ EIdentifier "K" ]
      , [ ESymbol Accent "\8594" ]
      , [ EIdentifier "L" ]
      ]
    ]
]
>>> 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">
        <mi>H</mi>
      </mtd>
      <mtd columnalign="center" style="text-align: center">
        <mo>→</mo>
      </mtd>
      <mtd columnalign="center" style="text-align: center">
        <mi>G</mi>
      </mtd>
    </mtr>
    <mtr>
      <mtd columnalign="center" style="text-align: center">
        <mo>↓</mo>
      </mtd>
      <mtd columnalign="center" style="text-align: center">
        <mspace width="3.0em" />
      </mtd>
      <mtd columnalign="center" style="text-align: center">
        <mo>↓</mo>
      </mtd>
    </mtr>
    <mtr>
      <mtd columnalign="center" style="text-align: center">
        <mi>K</mi>
      </mtd>
      <mtd columnalign="center" style="text-align: center">
        <mo>→</mo>
      </mtd>
      <mtd columnalign="center" style="text-align: center">
        <mi>L</mi>
      </mtd>
    </mtr>
  </mtable>
</math>