return null;
    public TopicMapObject getInstance() throws JellyTagException {        if (instance != null)            return instance;        if ((instance = getTopicFromContext(null)) != null) {            return instance;        }        if ((instance = getAssociationFromContext()) != null) {            return instance;        }        if ((instance = getOccurrenceFromContext()) != null) {            return instance;        }        return null;    }