how to declare an immutable class?

[Charles Hixson via Digitalmars-d-learn]

I want to declare a class all instances of which will be immutable, and 
all references to which will be inherently immutable (so that I don't 
need to slip a huge number of "immutable" statements in my code).

This is surely possible, because string acts just that way, but I can't 
figure out how to do this.

immutable class    Msg  {    this(...) immutable{...} ... }

doesn't work that way, as when I do

Msg m = new Msg (...);

I get:

Error: incompatible types for ((this.m) - (m)): 'immutable(Msg)' and 
'cellram.Msg'

and

Error: immutable method cellram.Msg.this is not callable using a mutable 
object


Does anyone know the correct approach?