Nitpick: "expensive" may be the wrong word there, to me that implies some kind of runtime cost. The stated reason for the keyword is that it forces users to thi

Nitpick: "expensive" may be the wrong word there, to me that implies some kind of runtime cost. The stated reason for the keyword is that it forces users to think a little bit more about mutability. You may want do this because safe mutability requires exclusive ownership.