Constthe package does not define the class, it only extends or relates it (a setIs, or a contains naming another package's class)
the record is a setClassUnion, so its SigClass supers are the classes it unites (its subclasses), not the ones it extends. Always ClassProp.Virtual too.
the class cannot be instantiated: representation("VIRTUAL"), contains = "VIRTUAL", or S7 abstract = TRUE
class-level boolean properties, packed into SigClass's bitfield