code: public static SortedSet<ClassLabel> getClassLabels(Database database) {
      final Relation<ClassLabel> relation = database.getRelation(TypeUtil.CLASSLABEL);
      return getClassLabels(relation);
    }
docs: Retrieves all class labels within the database.
  
  @param database the database to be scanned for class labels
  @return a set comprising all class labels that are currently set in the
  database