Can in a Service Node to Service Node communication an `as:context` be a Service Result

[phochste]

This is a scenario from @mielvds ErfgoedPod prototype use case. In a abstract from it can be summarized as:

• Institution A offers an artifact X to a service node S1
• The service node S1 does some processing on X and creates a service result Y
• The service node S1 offers the service result Y to a service node S2 for futher processing
• The service node S2 does some processing on Y and creates a service result Z
• The service node S2 announces service result Z to S1 with context service result Y
• The service node S1 announces service result Y to A with the context artifact X

The specification 0.0.1 specifies that notifications are 'artifact centric' and as:context must be interpreted as an artifact [1]. This would clash with the scenario above where S2 and S1 use it for service results.

[1] https://www.eventnotifications.net/#the-artifact-context