//@ For this check to be performed, `closure` is a *guard*: Rather than a normal
//@ reference, `borrow_mut` returns a smart pointer ([`RefMut`](https://doc.rust-
//@ lang.org/stable/std/cell/struct.RefMut.html), in this case) that waits until is
//@ For this check to be performed, `closure` is a *guard*: Rather than a normal
//@ reference, `borrow_mut` returns a smart pointer ([`RefMut`](https://doc.rust-
//@ lang.org/stable/std/cell/struct.RefMut.html), in this case) that waits until is