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$\begin{array}{|cc|}\hline a& b\\ x& y\\ \hline\end{array}$

Source Code:

    <math>
      <mtable frame="solid" width="auto">
        <mtr>
          <mtd>
            <mi>a</mi>
          </mtd>
          <mtd>
            <mi>b</mi>
          </mtd>
        </mtr>
        <mtr>
          <mtd>
            <mi>x</mi>
          </mtd>
          <mtd>
            <mi>y</mi>
          </mtd>
        </mtr>
      </mtable> 
    </math>