R help - Apr 2012 - to know observations and groups used in a mixed model using gee package

Does anyone know how to find the number of observations and groups used 
in a mixed model with gee package?
The summary function in gee package does not provide this information.
e.g.
data(warpbreaks)
summary(gee(breaks ~ tension, id=wool, data=warpbreaks, 
corstr="exchangeable"))

I much appreciate any help,

David Oseguera Montiel
Phd student Wageningen University