[Xy-pic] Background colour

[Damjan Perenic]

I am new to xy-pic and I quite like it. But I hit one problem which I
cannot figure out how to do it. I spent (way too) many hours searching
the web and trying various combinations, but failed.

I have following code:

 \begin{displaymath}
   \entrymodifiers={=<0.5pc>++[o][F-]}
   \xymatrix at C=1pc @R=3pt {
     *{r} & *{s} & *{t} & *{u} \\
     \infty & *++[F*:yellow]\txt{0} & \infty & \infty \\
     *+<1pc>{} \\
    \infty & \infty & \infty & \infty \\
     *{v} & *{w} & *{x} & *{y}
   }
 \end{displaymath}

Here is the output of the above block:
http://dl.dropbox.com/u/5378579/xy-pic.png

Where you see zero, I would like to get a circle like elsewhere, just
background should be Yellow.

If I change *++[F*:yellow]\txt{0} to *++[Fo*:yellow]\txt{0}, I get
just a plain 0, without any circle at all.
If I change it to *++[Fo:yellow]\txt{0}, I get a yellow circle, but as
expected, with a white background.

I am using pdfTeX, Version 3.1415926-1.40.11 and Xy-pic version 3.8.2.

I would appreciate if somebody point me out how to make a circle with
yellow background.

Thank you in advance.


Damjan