# Natural transformation arrow

I'm trying to do the following with xymatrix:

This is a diagram I drew sometime ago... But now I don't have the source and I can't remember how I did it, nor can I find an easy, simple way to do it.

More specifically, what I forgot how to do is the vertical arrow in the middle.

-
Are you interested in other ways of drawing such diagrams, for example in TikZ? If yes, we could show how this example would look with TikZ, or PSTricks. –  Stefan Kottwitz Jan 21 '12 at 9:54
@Stefan: Personally, I use xymatrix and I'm happy with it, but perhaps it is valuable to the community that the question be answered most completely; I'll leave it to your judgement. Thank you for the interest. –  lentic catachresis Jan 21 '12 at 12:42
I just wanted to show you possibilities, and well readable syntax and fine output of TikZ. No need to extend if you don't really wish it. –  Stefan Kottwitz Jan 21 '12 at 12:48

The 2cell module is what you need:

\documentclass{article}
\usepackage[all,cmtip,2cell]{xy}
\UseTwocells

\begin{document}
$\xymatrix@C+2pc{ \mathsf{CRing} \rtwocell^{\mathsf{GL}_n}_{U}{\;\;\;\det} & \mathsf{Grp} }$
\end{document}

-
I really used this? My (lack of) memory amazes me. Thank you, this is nice. However, how could I get the arrows to curve a little bit more? –  lentic catachresis Jan 21 '12 at 0:55
@BrunoStonek \rtwocell<6>...; page 45 of xyrefer.pdf –  egreg Jan 21 '12 at 1:00
Thank you very much! –  lentic catachresis Jan 21 '12 at 1:02