I want to differentiate a function $C(h,u)$ associated with Manning friction:
$$ C(h,u) = h^{-1/3} u \lvert u \rvert $$
According to WolframAlpha, $\mathrm{d} C / \mathrm{d} (hu)$ is $$ \frac{\mathrm{d}}{\mathrm{d}(hu)} h^{-1/3}u\lvert u \rvert = h^{-4/3} \lvert u \rvert $$
But how is this derivative calculated?