@Override
        String separator(Deque<SerializerWrapper> state) {
            if (state.peekFirst().subItems == 1) {
                return ") AND !(";
            } else {
                return " OR ";
            }
        }
Serialize Vespa query trees to YQL+ strings.

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