"The abstract supertype of all types with a well-defined
 notion of identity. Values of type `Identifiable` may be 
 compared using the `===` operator to determine if they are 
 references to the same object instance.
 
 For the sake of convenience, this interface defines a 
 default implementation of value equality equivalent to 
 identity. Of course, subtypes are encouraged to refine this 
 implementation."
by ("Gavin")
shared interface Identifiable {
    
    "Identity equality comparing the identity of the two 
     values. May be refined by subtypes for which value 
     equality is more appropriate. Implementations must
     respect the constraint that if `x===y` then `x==y` 
     (equality is consistent with identity)."
    shared default actual Boolean equals(Object that) {
        if (is Identifiable that) {
            return this===that;
        }
        else {
            return false;
        }
    }
    
    "The system-defined identity hash value of the instance. 
     Subtypes which refine `equals()` must also refine 
     `hash`, according to the general contract defined by 
     [[Object]]."
    see (`function identityHash`)
    shared default actual Integer hash => identityHash(this);
    
}