binary_sub.rs

struct Foo {
    value: uint
}

impl Sub<Foo, Foo> for Foo {
    fn sub(&self, rhs: &Foo) -> Foo {
        Foo {value: self.value - rhs.value}
    }
}

fn main() {
    let mut foo1 = Foo {value: 5};
    let foo2 = Foo {value: 2};
    foo1 -= foo2;
    println(fmt!("%?", foo1));
}