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\begin{document}



.... the following diagram commutes

\begin{picture}(70,70)(-30,0)
\put(110,55){${\cal{F}}(X_1)$}
\put(150,60){\vector(1,0){70}}
\put(230,55){${\cal{G}}(X_1)$}

\put(128,45){$\vector(0,-1){30}$}

\put(110,0){${\cal{F}}(X_2)$}
\put(150,5){\vector(1,0){70}}
\put(230,0){${\cal{G}}(X_2)$}

\put(245,45){$\vector(0,-1){30}$}
\end{picture}

we call .....

\end{document}