Global object made singleton.
boolean field isPhantom added.
Field origin added; only used for sub-objects.
ObjectNode made abstract; subclasses InternalObject, PhantomObject, and
GlobalObject introduced.
isGlobal() added as abstract method in ObjectNode and as concrete method
in its subclasses. Also, attribute 'field' added to phantom objects that
are subobjects.
Frame.mapsTo() added.