Function sub_frule 

pub fn sub_frule<S: ScalarAd>(x: S, y: S, dx: S, dy: S) -> (S, S)

Forward rule for sub.

Returns (primal, tangent).

Examples

use chainrules::sub_frule;

let (y, dy) = sub_frule(5.0_f64, 2.0_f64, 0.3_f64, 0.1_f64);
assert_eq!(y, 3.0_f64);
assert!((dy - 0.2_f64).abs() < 1e-12);