AggregatedEntityClass[class, "prop" -> f]
AggregatedEntityClass[class, {"prop1" -> f1, "prop2" -> f2, …}]
AggregatedEntityClass[class, propspec, gprop]
AggregatedEntityClass[class, propspec, "pname" -> f]
AggregatedEntityClass[class, propspec, {gspec1, gspec2, …}]