I must admit I still do not understand your classification; it's probably that I lack some background but unfortunately I consequently have a hard time understanding this system.

My *feeling* is that `imm` would represent bit-wise immutable types, whilst the two others would stand for what they are today; but in this case once again we fall on the issue that this proposal is about the interface of a type and not its implementation: logical const-ness vs bit-wise const-ness.