`\begin{subarray}` is not implemented

[tmke8]

This:

\sum_{\begin{subarray}{c}
   0 \le i \le m\\
   0 < j < n
\end{subarray}}

Should be rendered as:

<munder>
  <mo>∑</mo>
  <mtable displaystyle="false" scriptlevel="1">
    <mtr>
      <mtd style="padding: 0;">
        <mrow><mn>0</mn><mo>≤</mo><mi>i</mi><mo>≤</mo><mi>m</mi></mrow>
      </mtd>
    </mtr>
    <mtr>
      <mtd style="padding: 0;">
        <mrow><mn>0</mn><mo>&lt;</mo><mi>j</mi><mo>&lt;</mo><mi>n</mi></mrow>
      </mtd>
    </mtr>
  </mtable>
</munder>

The important things here are the displaystyle, scriptlevel and the padding: 0.

EDIT: actually, displaystyle="false" is default for tables, so I don't need that