"Observe" assertions
The protocol for interaction with a dataspace entity is very simple: any assertion can be sent to a dataspace. The job of a dataspace is to relay assertions on to interested peers; they do not generally interpret assertions themselves.
The sole exception is assertions of interest in other assertions.
These are called "Observe" assertions, or subscriptions:
Observe = <Observe @pattern dataspacePatterns.Pattern @observer #:any>.
An Observe assertion contains a pattern and a reference to an
observer entity. When an Observe assertion is published to a dataspace, the dataspace alters
its internal index to make a note of the new expression of interest. It also immediately relays
any of its already-existing assertions that match the pattern to the observer entity. As other
assertions come and go subsequently, the dataspace takes care to inform the observer entity in
the Observe record of the arrival or departure of any of the changing assertions that match
the pattern.