Mutability or not doesn't change the fundamental assertion that the result is not intuitive even to the author of the class hierarchy. My whole point is not whe

Mutability or not doesn't change the fundamental assertion that the result is not intuitive even to the author of the class hierarchy. My whole point is not whether the modeling of the problem in a typical type system is correct or not. The point is that a typical type system allows for a wide range of "incorrectness" that still compiles and runs and does exactly what it was designed to do but still leaves the programmer with an inscrutable result. Therefore, it can be seen, that there might be other, better ways to assert your system is correct than using types.