A Packer's unpack() function receives the raw packed JSON value as its first argument and this registration function as the second. Functions registered with WhenDone will be run last-in-first-out (stack order) when the raw JSON has been processed into the final object instances. WhenDone functions are needed whenever the packed data is more complex than simple JSON values, for example when referential equality needs to be maintained or when the pack function returns anything that needs to be re-packed by some other packer. Referenced objects may not be fully initialized when the registered function is called, but its instance will be instantiated so that references can be fixed.