@Override
    public ListIterator<SpanNode> childIterator() {
        return new InvalidatingIterator(this, children().listIterator());
    }
Traverses all immediate children of this SpanList. The ListIterator returned support all optional operations
specified in the ListIterator interface.

@return a ListIterator which traverses all immediate children of this SpanNode
@see java.util.ListIterator
document/src/main/java/com/yahoo/document/annotation/SpanList.java