Set range for a single pdf in RooFit

Dear ROOTers,

as described in the UserGuide, it is possible to set a range or composite of ranges for a fit.
However, I would like to fit with a RooAddPdf model:

RooAddPdf model(“model”,“model”,RooArgList(gauss,chebychev, exponential),RooArgList(nsig,nbkg, nexp)) ;

but to constrain ONLY the range for the exponential pdf (so that it would fit e.g. only data in a region left of the gaussian peak), while fitting the gaussian and polynomial pdf in full range.

How is this done?

Many thanks for your help,