Create the Subscriber or VirtualPublisher that consumes the incoming stream, plus the materialized value. Since Subscriber and VirtualPublisher do not share a common supertype apart from AnyRef this is what the type union devolves into; unfortunately we do not have union types at our disposal at this point.

public abstract Object Create(
	MaterializationContext context,
	out TMat materializer
)

abstract Create : 
        context : MaterializationContext * 
        materializer : 'TMat byref -> Object