Derivative notation

[intervoxel]

What is the meaning of

\frac{\partial F(x)}{\partial(\partial f(x)/\partial x)}

in which a derivative appears in place of the variable x in the denominator?

Anyone, please?

[mathman]

Let u = ∂f(x)/∂x. Then the expression is ∂F(x)/∂u. In order to make sense out of it you need to know F(x) and f(x).

[intervoxel]

Thank you.