public InstantiationStack(IList <PvmActivity> activities, PvmActivity targetActivity, PvmTransition targetTransition)
{
EnsureUtil.ensureOnlyOneNotNull("target must be either a transition or an activity", targetActivity, targetTransition);
this.activities = activities;
// TODO: make this a subclass that contains targetActivity and targetTransition?!
this.targetActivity = targetActivity;
this.targetTransition = targetTransition;
}