OTS — Proposal to support interposition (Active Service spec orbos/2000-06-19)

The problem is that, in domain 2 where there is no SignalSet, how are the
Outcomes from Action1 and Action2 supposed to be combined and sent back as
a single Outcome to the superior ActivityCoordinator?
Further, even if the appropriate SignalSet did exist in domain 2, how could
the subordinate ActivityCoordinator get it involved in the request from the
superior - the interposed ActivityCoordinator is merely acting as a global
Action registered with its superior.

I propose the following in order to support interposition:
1. Any signal received by a subordinate ActivityCoordinator be delegated to
a SignalSet (assumed to be local since we're using interposition). If the
SignalSet is not available, then return a standard Outcome (that also needs
to be specified) that indicates that the subordinate ActivityCoordinator is
not interested in any further signals from this particular broadcast.
2. Extend the CosActivity::SignalSet interface to provide a
set_signal(signal) method to enable a subordinate ActivityCoordinator to
indicate to the SignalSet that it will be used only to process the
specified Signal (and the Outcomes that are subsequently produced by
Actions that receive the Signal) and not to actually produce any Signals.
Only the root SignalSet should be producing Signals.

An alternative would be to split the SignalSet into 2 objects - the
function of 1 object being to produce Signals and the other to combine
Outcomes. The down-side of this is the additional complexity it introduces
in terms of additional flows between the 2 parts of the SignalSet in the
root.
Another alternative would be to specialize the SignalSet with a
SubordinateSignalSet.

Update the specification to incorporate a new entity, the SubordinateSignalSet