Mutable reference for immutable AA

[Jack Applegame via Digitalmars-d-learn]

I need mutable storage for immutable associative array. Just 
create new immutable AA and store it for future passing it 
between threads/fibers.

First attempt: just immutable AA
immutable aa = ["1":1, "2":1];
aa = ["1":1, "2":1]; // fail, can't assign a new AA

Second attempt: mutable AA with immutable elements.
immutable (int)[string] aa = ["1":1, "2":1];
aa = ["1":1, "2":1]; // sucess!
aa["2"] = 2;         // doesn't compile, is AA immutable?
aa.remove("1");      // fail, this compiles, AA actually isn't 
immutable

Third attempt: Rebindable
Rebindable!(immutable int[string]) aa; // fail, Rebindable 
doesn't accept AAs