code: private void clearAll(DBIDs ids, WritableDataStore<HashSetModifiableDBIDs> sets) {
      for(DBIDIter it = ids.iter(); it.valid(); it.advance()) {
        sets.get(it).clear();
      }
    }
docs: Clear (but reuse) all sets in the given storage.
  
  @param ids Ids to process
  @param sets Sets to clear