private void onInvokingFill(Searcher searcher, String summaryClass) {
        int traceFillAt = 5;
        if (trace().getTraceLevel() < traceFillAt) return;
        trace().trace("Invoke fill(" + summaryClass + ") on " + searcher, traceFillAt);
    }
Calls fill on the next searcher in this chain. If there is no next, nothing is done.
container-search/src/main/java/com/yahoo/search/searchchain/Execution.java