RefCopy a reference to the copy of a value

superclass: Ref

A Ref instance is an object with a single slot named 'value' that serves as a holder of an object.

RefCopy, in difference to Ref, returns only copies of the value when next is called.

This can be useful when the original is to be kept unchanged.

see Ref for other methods.

// example:

a = [1, 2, 3];

x = RefCopy(a);

b = x.next;

b.put(0, 100); // modify b

a; // a is unchanged.