@Override
    public void write(AddValueUpdate update, DataType superType) {
        writeValue(this, ((CollectionDataType)superType).getNestedType(), update.getValue());
        putInt(null, update.getWeight());
    }
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