Function powf_rrule 

pub fn powf_rrule<S: ScalarAd>(x: S, exponent: S::Real, cotangent: S) -> S

Reverse rule for powf with fixed exponent.

Examples

use chainrules::powf_rrule;

let dx = powf_rrule(2.0_f64, 3.0, 1.0);
assert_eq!(dx, 12.0);