return fmt.Errorf("reverse select %v wanted %v. got %v.", key, want, got)
		}
		return nil
	}
}

func emptyMap(m *BiMultimap) error {
	if len(m.labeledObjects) != 0 {
		return fmt.Errorf("Found %v labeledObjects. Wanted none.", len(m.labeledObjects))
	}
	if len(m.selectingObjects) != 0 {
		return fmt.Errorf("Found %v selectingObjects. Wanted none.", len(m.selectingObjects))
	}
	if len(m.labeledBySelecting) != 0 {