// rustc -V
// rustc 1.2.0-nightly (0cc99f9cc 2015-05-17) (built 2015-05-18)

use std::ops::Add;

fn foo<Num>(n: Num) -> Box<FnMut(Num) -> Num>
        where Num: Add<Output=Num> + Copy + 'static {
    let mut acc = n;
    Box::new(move |i: Num| {
        acc = acc + i;
        acc
    })
}

fn main() {
    let mut x = foo(1.);
    x(5.);
    foo(3.);
    println!("{}", x(2.3));
}