String separator(Deque<SerializerWrapper> state) {
            throw new UnsupportedOperationException("Having several items for this query operator serializer, "
                                                    + this.getClass().getSimpleName() + ", not yet implemented.");
        }
Serialize Vespa query trees to YQL+ strings.

@author Steinar Knutsen
container-search/src/main/java/com/yahoo/search/yql/VespaSerializer.java