readonly?

[Namespace]

Maybe D need's a readonly keyword.
Sometimes i have a class which can take an object from 
everywhere to store it. So it can not be const, because i didn't 
just initialized it with a ctor.
But i don't want to change the object, i only want to read or 
call const methods. What now?
I'd suggest a readonly keyword for that.

[code]
class Foo {
readonly:
     Bar _b;

public:
void SetBar(readonly Bar b) {
     _b = b;
}
}
[/code]

Or has D an alternative?