[R] cross-classified random factors in lme without blocking

[Gordon Smyth]

On page 165 of Mixed-Effects Models in S and S-Plus by Pinheiro and Bates 
there is an example of using lme() in the nlme package to fit a model with 
crossed random factors. The example assumes though that the data is 
grouped. Is it possible to use lme() to fit crossed random factors when the 
data is not grouped?

E.g., y <- rnorm(12); a=gl(4,1,12); b=gl(3,4,12). Can I fit an additive 
model like y~a+b but with a and b random?

Everything I've tried gives an error:

 > lme(y~1,random=~1|(a+b))
Error in switch(mode(object), name = , numeric = , call = object, character 
= as.name(object),  :
         [[ cannot be of mode (
 > lme(y~1,random=~a+b)
Error in getGroups.data.frame(dataMix, groups) :
         Invalid formula for groups

Thanks
Gordon