What is the “existing Type System”? The one in the Mellor-Balcer book? Or (since you included structures) the one currently in BridgePoint?
I agree with Sean, but I need some clarification on the external type suggestion. If we go by the Mellor-Balcer types, then we have ordinals and collections (IIRC). The Data Types in OOA paper has some discussion on ordinal, but doesn’t include collections. Would an external type for an ordinal have accessible elements if the type was known in the original domain or must it always be atomic? Collections should always be atomic.