private Pong invokePing(Ping ping, Searcher next) {
        Pong annotationReference;
        if (next instanceof PingableSearcher) {
            annotationReference = ((PingableSearcher) next).ping(ping, this);
        } else {
            annotationReference = ping(ping);
        }
        return annotationReference;
    }
Calls ping on the next search in this chain. If there is no next, a Pong is created and returned.
container-search/src/main/java/com/yahoo/search/searchchain/Execution.java