head const (again), but for free?

[jmh530]

On Wednesday, 13 January 2021 at 19:42:14 UTC, Meta wrote:
> [snip]
>
> Is there even any value to having head-const in a language? As 
> I think Walter has said before, it's basically just 
> documentation/convention in C++. I can see the value in 
> head-immutable, in terms of type system guarantees, but not 
> head-const.

I don't really understand the difference between head-const and 
head-immutable.