@Override
    public void write(SpanList spanList) {
        buf.put(SpanList.ID);
        buf.putInt1_2_4Bytes(spanList.numChildren());
        Iterator<SpanNode> children = spanList.childIterator();
        while (children.hasNext()) {
            write(children.next());
        }
    }
Write out the value of weighted set field

@param field - field description (name and data type)
@param ws    - field value
document/src/main/java/com/yahoo/document/serialization/VespaDocumentSerializer6.java