BPMN21 — meta-model doesn't provide way to serialize intermediate catch events attached to participant bands of choreography task

On pp. 341~343 the specification details some intermediate catch events that can be attached to activity boundaries, namely Message, Timer, Cancel, Compensation, Conditional, Signal and Multiple. This is similar to attaching events to activities in Process diagrams. The meta-model supports attaching of events to activity boundaries in Processes through the BoundaryEvent element. However, no similar element is available for Choreography Activities. Instances of BoundaryEvent obviously cannot be used (because ChoreographyActivity does not extend Activity). Therefore a new Class should be added. This class (called e.g. ChoreographyBoundaryEvent) should (1) have a reference to the ChoreographyActivity to which the event is attached, and (2) depending on the type of event, specify to which of the ParticipantBands the event is attached. This is necessary because some events, namely Message, Cancel and Compensation are "local" (i.e. visible) to only one of the Participants involved in the ChoreographyActivity. The specification should clearly state in which cases this reference to a Participant is compulsory, and in which cases optional. For example, it might be optional in the case of Signal events, to symbolize that the event is trigger by the reception of the signal by one particular participants (which is different than the "firing" of the signal, given the fact that the specification does not seem to require signals to be "immediately" received, but just "eventually" received by the participants).