LaTeX-examples/documents/GeoTopo/figures/torus-invalid-triangulation-2.tex

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    \coordinate (cd) at ($(c)!0.5!(d)$);
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    \coordinate (right-intersection) at ($(m)!0.5!(c)$);
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        \draw[postaction={decorate}] (a.center) -- (b.center);
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    \begin{scope}[decoration={
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    \draw (a.center) -- (c.center);
    \draw (b.center) -- (d.center);
\end{tikzpicture}