rebind of const class variables

[qqiang via Digitalmars-d-learn]

I am writing a tree data structure, and I have the following code:

```D
final class Node {
     private {
	int val_;
	Node parent_;
	Node left_;
	Node right_;
     }

     @property
     const(Node) maximum() const {
	auto ret = this;
			
	while (ret.right_) {
	    ret = ret.right_;
	}
			
	return ret;
     }
}
```

It failed to compile and complaint that `cannot modify const 
expression ret`。

Since `ret` is just a binding to a const class object, why can't 
I rebind it to another const class variable?

Must I use pointers to cope with this?

Thx