OTS — Is support for registering an Action with multiple SignalSets needed?

P23: "An Action may register interest in more than one SignalSet and an
Activity may use more than one SignalSet during its lifetime."
P50: "An Action is implicitly associated with a single Activity and should
only be with that Activity. An Action may receive many different Signals
from different SignalSets."

The interface to register an Action with an Activity is add_action, on
either the ActivityCoordinator or Current interfaces. It is defined:
void add_action(in Action act, in string signal_set_name, in long priority)
raises(SignalSetUnknown);

If an Action is to be registered with an interest in more than one
SignalSet, then it must be passed as a parameter on multiple add_action
calls. Given that the specification implies that calling
ActivityCoordinator.add_action(TheAction, "someSS", priority)
multiple times is OK, the behaviour needs to be defined for the case where
TheAction is added multiple times to the same SignalSet. In particular,
what if the priority is different on multiple add_action calls?
At the very least I believe a new exception - ActionAlreadyRegistered? -
needs to be thrown in the case where an Action is registered multiple times
with the same SignalSet.

I question the need to support registering an Action with multiple
SignalSets at all. I propose that this be illegal as I believe there is no
need for it. The new exception is still required.

see above