How does "immutable" work.

[egslava]

Hello! Sorry for my English.

I read manual about "immutable" and "const" keyword:
http://dlang.org/const3.html

And tried to build my program:
http://dpaste.dzfl.pl/f803ae94

If I will change "immutable" to "const" output will not changed.
But why? Why output is look like this?

I would understand the output like this:
10
10
7FFF7E68AEB0
7FFF7E68AEB0

In this case, I would think - we just ignoring all assign 
operators.

I would understand the output like this:
10
20
7FFF7E68AEB0
FFFFFFFFFFFFF

In this case, I would think - we just one more time located 
memory and copied value of variable.

But why the output does look like this? How does this 
construction work? Why memory addresses are same, assign is 
working, but value of immutable variable doesn't change?

I would even think, that in compile-time compiler changes
writeln(x) to writeln(10), but why we can dereference "x"?