code: public final int addChild (Task<E> child) {
  		int index = addChildToTask(child);
  		if (tree != null && tree.listeners != null) tree.notifyChildAdded(this, index);
  		return index;
  	}
docs: This method will add a child to the list of this task's children
  
  @param child the child task which will be added
  @return the index where the child has been added.
  @throws IllegalStateException if the child cannot be added for whatever reason.