public ListIterator<SpanNode> childIterator(int i) {
        return childTrees.get(i).childIterator();
    }
Traverses all immediate children of the given subtree of this AlternateSpanList.
The ListIterator returned supports all optional operations
specified in the ListIterator interface.

@param i the index of the subtree to iterate over
@return a ListIterator which traverses all immediate children of this SpanNode
@see java.util.ListIterator
document/src/main/java/com/yahoo/document/annotation/AlternateSpanList.java