public static void main(String[] args) {
	    ObjectStructure o=new ObjectStructure();
	    o.attach(new ConcreteElementA());
	    o.attach(new ConcreteElementB());
	    ConcreteVisitor1 v1=new ConcreteVisitor1();
	    ConcreteVisitor2 v2=new ConcreteVisitor2();
	    o.accept(v1);
	    o.accept(v2);
	}