LOG.info(">> merging a list of " + lists.size() + " partial lists");
if (lists.size() > 1) {
PostingsListDocSortedPositional merged = new PostingsListDocSortedPositional();
PostingsListDocSortedPositional.mergeList(merged, lists, collectionDocumentCount);
lists.clear();
mergedList.add(PostingsListDocSortedPositional.create(merged.serialize()));
context.getCounter(Reduce.Merges).increment(1);
} else {
PostingsList pl = lists.remove(0);
pl.setCollectionDocumentCount(collectionDocumentCount);
mergedList.add(pl);