[R] display functions in groupedData and lme

[Scott Rifkin]

I'm trying to set up a mixed model to solve using lme.  It will have 3 
fixed effects, two random effects and two interaction terms. 

I've been reading Pinheiro's and Bates's book on the nmle library, but
find the part about display functions to be unclear.  When creating a 
groupedData object from a data.frame, you need to enter a function of the 
form: response ~primary|grouping

Does this display function serve any purpose later on when specifying the 
model in the call to lme?  If so, how would I include more terms in 
primary or grouping (I tried primary1+primary2+... as per one post in the 
archives, but it didn't work).  If not, is it just for purposes of 
displaying the data?

Thanks much,
Scott Rifkin