| marker class for indicating that a PG is unlikely to ever be filled.
The "null" version of a PG is available only via the static data member
WhateverPG.nullPG, where WhateverPG is the base PG interface.
If an Asset has an actual null as the value for a given PG, it is
interpreted as unspecified , and potentially available for
late-binding. A value of a Null_PG instance prevents late-binding,
activation.
All accessors will throw an UndefinedValueException if called.
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