prepare.rs

    fn prepare<A: Clone + 'static, B: Clone, C, AB: Fn(A, B) -> C + 'static>(op: AB) -> Box<dyn Fn(A) -> Box<dyn Fn(B) -> C>> {
        let op_rc = Rc::new(op);
        Box::new(
            move |a: A| {
                let a_rc = Rc::new(a.clone());
                let op_rc = op_rc.clone();
                Box::new(move |b: B| {
                    let a = a_rc.as_ref();
                    let op = op_rc.as_ref();
                    op(a.clone(), b.clone())
                })
            }
        )
    }