\( \left. \frac{\partial W(x, \; ...)}{\partial x} \right|_{\text{varying x, constant everything else}} = \left. \frac{\partial W(r(x, \; ...), \; ...)}{\partial r} \right|_{\text{varying r(x), constant everything else}} \left. \frac{\partial r(x, \; ...) }{\partial x} \right|_{\text{varying x, constant everything else}} \)
