void retrieveMoreHits(DocumentGroup documentGroup) {
            if ( ++numRetrieveMoreHitsCalls <
                 parameters.maxTimesRetrieveHeterogeneousHits) {

                retrieveHeterogenousHits();

                if (numHits(documentGroup) <
                    documentGroup.targetNumberOfDocuments) {

                    retrieveMoreHits(documentGroup);
                }

            } else {
                retrieveRemainingHitsForGroup(documentGroup);
            }
        }
Groups hits according to document type.
For each group, the desired number of hits can be specified.

@author Tony Vaagenes
container-search/src/main/java/com/yahoo/prelude/searcher/MultipleResultsSearcher.java