code: @Override
  	protected int addChildToTask (Task<E> child) {
  		if (this.rootTask != null) throw new IllegalStateException("A behavior tree cannot have more than one root task");
  		this.rootTask = child;
  		return 0;
  	}
docs: This method will add a child, namely the root, to this behavior tree.
  
  @param child the root task to add
  @return the index where the root task has been added (always 0).
  @throws IllegalStateException if the root task is already set.