borrowed_closure

fn double(f: &|int| -> int, a: int) -> int {
    (*f)(a) * 2
}
fn borrow(f: &|int| -> int) -> int {
    let r = double(f,1);
    (*f)((*f)(r))
}

fn main() {
    print!("{}", borrow(&|a:int| a+1));
}

In &|a: int| a + 1, the type annotation for a is optional; |a| a + 1 would work just as well.