pub fn identity(s: &str) -> &str {
    s
}

pub fn apply1<F>(v: &str, f: F) -> &str where F: Fn(&str) -> &str {
    f(v)
}

pub fn apply2<T, R, F>(v: T, f: F) -> R where F: Fn(T) -> R {
    f(v)
}

#[test]
pub fn test_apply1() {
    assert_eq!("test", apply1("test", identity));
}

#[test]
pub fn test_apply2() {
     assert_eq!("test", apply2("test", identity));
}