"numerical derivatives" should probably say "numerical partial derivatives"

[tmccombs]

In the chapter on "Variables and Functions" in the section on "Inference of Labeled and Optional Arguments" it says

Consider the following example for computing numerical derivatives of a function of two real variables

It would be more precise to either say that it computes the partial derivatives with respect to x and y, or that it computes the gradient (which is a vector composed of those partial derivatives).