Explore potential applications for a macro that maps variables to a gradient

[FL03]

It may be possible to generate a macro that represents the gradient;

extern crate acme;

use acme::operator;

fn main() {
    let dx = mul_gradient!(x: 10f64);

    let (x, y) = (3f64, 4f64);
    let dx = mul_gradient!(x);
    let dy = mul_gradient!(y);

    assert_eq!(dx, y);
    assert_eq!(dy, x);
}

#[operator]
pub fn mul<A, B, C>(x: A, y: B) -> C where A: core::ops::Mul<B, Output = C> {
    x * y
}