public class MutableDouble
implements Valuable, java.lang.Cloneable
MutableDouble simply holds a double value, which can be changed at any time.
Can't get much simpler than a class like this! Note that this class is not synchronized,
and shouldn't be used in a multithreaded environment without a lock.
Note that MutableDouble hashes by ADDRESS and not by VALUE. Thus
(new MutableDouble(2)).equals(new MutableDouble(2)) is FALSE.