private static String decodeSummary(String summaryClass, FastHit hit, byte[] docsumdata, DocsumDefinitionSet docsumSet) {
        String error = docsumSet.lazyDecode(summaryClass, docsumdata, hit);
        // System.err.println("decoded, error? " + error);
        if (error == null) {
            hit.setFilled(summaryClass);
        }
        return error;
    }
Fills the hits.

@return the number of hits that we did not return data for, and an optional error message.
        when things are working normally we return 0.
container-search/src/main/java/com/yahoo/vespa/streamingvisitors/StreamingBackend.java