private String getProperty(Hit hit, String field) {
            if ("[id]".equals(field)) return hit.getId().toString();
            Object o = hit.getField(field);
            return o == null ? null : o.toString();
        }
Produce a single blended hit list from a group of hitgroups.

This assumes that all hits are organized into hitgroups. If not, blending will not be performed.
container-search/src/main/java/com/yahoo/prelude/searcher/BlendingSearcher.java