code: private Interaction findContext(String id) {
          for(int i= _currentInteractions.size() - 1; i >= 0; i--) {
              Interaction ia= (Interaction) _currentInteractions.get(i);
              
              if(ia.id.equals(id)) {
                  return ia;
              }
          }
          return null;
      }
docs: Caller must hold lock on interactions