Hi @albanD, thank you for the reply. It does make sense to me to take the derivative of a function w.r.t another function even if we are not talking about functionals. After all, every variable is an identity function of itself: f(x,y) = x + y = I(x) + I(y), where I(.) is the identity function. So, df(x,y)/dx is  just df(x,y)/dI(x). In the same vein, it does make sense to ask what df(x,y)/dg(x,y) is, where f(x,y) = x + y + xy = x + y + g(x,y), for example. The problem is that if the product xy changes, x and y could change individually or jointly, and there may be multiple ways by which this can happen for a given change in xy. This means that it’s not clear how to calculate df(x,y)/dg(x,y), since it’s not clear how to calculate dx/dg(x,y) and dy/dg(x,y) in the above example. It feels like a very simple problem and yet I’m at a loss.