[R] gnls or nlme : how to obtain confidence intervals of fitted values

[Pierre MONTPIED]

Hi

I use gnls to fit non linear models of the form y = alpha * x**beta 
(alpha and beta being linear functions of a 2nd regressor z i.e. 
alpha=a1+a2*z and beta=b1+b2*z) with variance function 
varPower(fitted(.)) which sounds correct for the data set I use.

My purpose is to use the fitted models for predictions with other sets 
of regressors x, z than those used in fitting. I therefore need to 
estimate y with (95%) confidence intervals.

Does any body knows how to do this with R ?

Thanks