+ vector<Row*> get_rows();
+ Row* get_row_by_id(size_t id);
+
+ vector<const AnnotationClass*> ann_classes() const;
+ vector<AnnotationClass*> ann_classes();
+ AnnotationClass* get_ann_class_by_id(size_t id);
+ const AnnotationClass* get_ann_class_by_id(size_t id) const;
+