@Override
    public ListIterator<SpanNode> childIterator() {
        List<ListIterator<SpanNode>> childIterators = new ArrayList<ListIterator<SpanNode>>();
        for (Children ch : childTrees) {
            childIterators.add(ch.childIterator());
        }
        return new SerialIterator(childIterators);
    }
Traverses all immediate children of all subtrees of this AlternateSpanList.
The ListIterator only supports iteration forwards, and the optional operations that are implemented are
remove() and set(). add() is not supported.

@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