# How to center twocell arrows

I'm using 2cell (as in this previous question of mine: Natural transformation arrow ).

I'd like to know how to center not just the twocell double arrow, but arrow with its name too. A picture is worth a thousand words:

This diagram is generated by the following code:

$\xymatrix@C+3pc{J\rtwocell<4>^{\Delta(C)}_{\Delta(C')}{\; \; \; \; \; \Delta(f)} & \C}$


I'd like to know how to center the whole $\Downarrow \Delta(f)$, not just the $\Downarrow$.

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Is TikZ also possible? –  Marco Daniel Apr 23 '12 at 17:59
@MarcoDaniel: I'd rather stick to xypic. –  lentic catachresis May 18 '12 at 2:45

I don't know whether this can be done with twocell, but with regular xypic you can certainly achieve it:

\documentclass{article}
\usepackage{amsmath}
\usepackage[all,cmtip]{xy}

\begin{document}
$\xymatrix@C+3pc{ J \ar@{}[r]|{\Downarrow \Delta(f)} \ar@/_/[r]_{\Delta(C)} \ar@/^/[r]^{\Delta(C)}& C }$

$\xymatrix{ J \ar@/_/[rr]_{\Delta(C)} \ar@/^/[rr]^{\Delta(C)}& {\Downarrow \Delta(f)} & C }$
\end{document}


Next time please post a complete MWE, even in follow-up questions.

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