Why can't function expecting immutable arg take mutable input?

[Shriramana Sharma via Digitalmars-d-learn]

Ali Çehreli wrote:

> Actually, others gave the answer to that question, which was apparently
> not very clear. :)

Yes it was clear and I did understand it: and I posted a reply thanking the 
others too, but for some reason it was still sitting in my outbox...

-- 
Shriramana Sharma, Penguin #395953