2.5 todo instead of unimplemented

[Chris00]

For Section 2.5, wouldn't it be better to use the todo! macro in each of the cases:

pub fn pop(&mut self) -> Option<i32> {
    match self.head {
        Link::Empty => {
            todo!()
        }
        Link::More(node) => {
            todo!()
        }
    }
}

or a todo!() at the end?