[Ifeffit] restraints in larch

Hello I'm trying to use larch to refine the coordination of Y in solution. I need to use 2xO paths to fit the first shell and would like to restrain the total coordination to be between 5.5 and 8. I can apply the constraint to each nO, but not the total, i.e. nO1 = param(7.0, min=5, max=7, vary=True), nO2 = param(1, vary=True), but ntot1 = param(expr='nO1+nO2', min=5.0, max=8.5) does not work. How can I restrain the total coordination? Also, having some problems with the gofs and errors - the chi square is rather small and often the errors are not reported and it is not clear why. Thanks Barb