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TikZ is not mandatory:-)
\documentclass{article}
\usepackage{color}
\makeatletter
\def\pdiag#1{{%
\setlength\unitlength{15pt}%
\begin{picture}(10,10)(-2,-2)%
\put(0,-2){\line(0,1){10}}%
\put(7,-2){\line(0,1){10}}%
\put(-2,-0){\line(1,0){10}}%
\put(-2,7){\line(1,0){10}}%
\put(7.2,7){S}%
\count...