\PGset[0.8em]
\begin{picture}( 7,8)

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\drawline(1,1)(6.6,6.6)

\put( 0.7, 0.2 ){$\scriptstyle A$}
\put( 6.0, 0.2 ){$\scriptstyle B$}
\put( 6.0, 6.7 ){$\scriptstyle C$}
\put( 0.7, 6.7 ){$\scriptstyle D$}


\end{picture}
\PGrestore