[Xy-pic] rotate labels

[Wolfhart Totschnig]

Hi,

I am new to xy-pic. I have a problem with rotating the labels in my 
diagram. For example, I have the following:

\ar[ur]_{causation} \ar@{-}[u]_{predication} & ...

I would like to have both label texts aligned with the arrows, that is, 
the one diagonally and the other vertically. How can this be done? I 
couldn't find the solution, neither in the guide nor in the reference 
manual nor in the archive of this list. Can someone help me out?

Thanks!
Wolfhart