When I retrieve an object of type ActivityInstance, there should be a method that returns the ID of a single execution. That should be the ID of the principal execution, which means:

• for instances of a scope activity, return the scope execution
• for instances of a non-scope activity, return the respective leaf execution (scope or concurrent)

or stated differenly: the execution that is used for invoking listeners and delegates given the current execution tree.

Why? This is useful when using the activity instance tree to identify an execution and then set a local variable or call RuntimeService#signalExecution. Currently, there is only ActivityInstance#getExecutionIds which is not clear to use for this purpose.