\usepackage {pstricks,pst-node}
\begin{psmatrix}
A & B \\
$\sqrt{\frac{x + y}{z}}$ & D
\ncline{1,1}{1,2}
\ncline{1,2}{2,2}
\ncline{2,2}{2,1}
\ncline{2,1}{1,1}
\end{psmatrix}
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