public Set<OWLClass> getSubClasses(String expression) {
	if (expression.trim().length() == 0) {
	    System.out.println("No class expression specified");
	    return Collections.emptySet();
	} else {
	    OWLClassExpression classExpression = this.parseClassExpression(expression.trim());
	    NodeSet<OWLClass> subClasses = this.getReasoner().getSubClasses(classExpression, true);
	    return subClasses.getFlattened();
	}
    }