in gazebo/src/cell.rs [282:289]
fn test_ref_map_dropping() {
let c = RefCell::new("test".to_owned());
let p = ARef::new_ref(c.borrow());
let p = ARef::map(p, |x| &x[1..3]);
assert_eq!(&*p, "es");
mem::drop(p);
assert!(c.try_borrow_mut().is_ok());
}