LK tag relationship checkpoint. There are accessibility checks which are not a function of one tag, but a relation. Charles depends on Len, Len depends on Wendy, Wendy depends on Michael, Michael depends on Charles. That cycle does or does not meet the requirement. You can define which roles various tags perform.
Man is mortal.
There are number of ways to do that basic logic. It's a data format. It doesn't include the processing. Take Yahoo, main navigation bar and another one. If use class no longer need map.