code: @Nullable
  	public Event nextInvalid() {
  		final Iterator<Entry<Integer, State>> iter = states.entrySet().iterator();
  		if (iter.hasNext()) {
  			final Entry<Integer, State> entry = iter.next();
  
  			State currentState = entry.getValue();
  			int address = entry.getKey();
  			iter.remove();
  
  			EventType event = currentState.randomInvalidTransition(rnd);
  			return new Event(event, address);
  		}
  		else {
  			return null;
  		}
  	}
docs: Creates an event for an illegal state transition of one of the internal
  state machines. If the generator has not yet started any state machines
  (for example, because no call to {@link #next(int, int)} was made, yet), this
  will return null.
  
  @return An event for a illegal state transition, or null, if not possible.