opCmp with structs

[Alex via Digitalmars-d-learn]

Ok, ok... I see the direction, but I still miss something:
 From the point of view of "Programming in D", chapter 33.3, 
"Immutability of the slice vs the elements":

I don't want to have an immutable slice but immutable elements. 
And your answers imply that sorting not only modify the slice 
itself but also mutate the elements. This is a little bit 
strange, in my opinion, as sorting modifies only relationships 
between the sorted elements not the elements itself. At least in 
my head, at least for the moment :)

A test says the same thing:

void main()
{
	immutable int[] immSlice = [2, 1];
	sort(immSlice);
}

and

void main()
{
	immutable(int)[] immSlice = [2, 1];
	sort(immSlice);
}

both give errors:
Error: template std.algorithm.sorting.sort cannot deduce function 
from argument types !()(immutable(int[]))
and
Error: template std.algorithm.sorting.sort cannot deduce function 
from argument types !()(immutable(int)[])
respectively.

So the question remains, how to sort? Do you think removing the 
immutable property from an id is the semantically right way? Or 
do you have a hint, maybe something like, a dup copy with 
removing immutability and returning a new, sorted copy? If you 
think the latter is the way to choose, how is the syntax there? ;)