[R] Problems using na.include

[Greg Distiller]

Hi
I am having problems using na.action=na.include in a nlme call. Basically I 
am trying to run a particular model without including any covariates but my 
dataset does include some covariates (factors) with missing values. My 
understanding from the Pinheiro and Bates book is that one would use 
na.action=na.include in order to treat the missing values as another factor 
level and then one can also use naPattern to specify which rows must be 
excluded from the calculation of the objective function.

I have tried this but keep getting error messages about being unable to find 
the object or the function "na.include". I have also tried to replicate the 
particular Phenobarbital example verbatim from the book but am getting the 
same error message. Could it be that my version of the nlme package is 
outdated? I also cannot find any help on na.include...

Thanks

Greg