extracting r-sq from lme model

I've been using the nlme library to construct lme models and I'd like to
extract an r-sq value for the model - how do I do this?

With glm I just used the reported (null deviance-deviance)/(null deviance).
Deviance is not reported directly for lme nor can it be extracted using the
deviance() function.  How do I work out the deviance or can I work out R-sq
in some other way?

Thanks,
Yvonne 

Yvonne Buckley
Department of Biology
Imperial College, Silwood Park
Ascot, Berkshire
SL5 7PY  
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