Alias variable from another class

[Begah via Digitalmars-d-learn]

I made a little program to illustrate my confusion :

   import std.stdio;

   class _1 {
	  int i;
	
	  this(int n) {
		  i = n;
	  }
   }

   class _2 {
	  _1 instance;
	
	  alias twelve = instance.i;
	
	  this() {
		  instance = new _1(12);
	  }
   }

   void main() {
	  _2 two = new _2();
	  writeln(two.instance.i);
	  writeln(two.twelve);
   }

What i tried to do is create an alias in class _2 of a variable 
in class _1.
What i would like is :
   two.twelve
to extends to:
   two.instance.i
But the compiler complains it cannot find "this" :
   Error: need 'this' for 'i' of type 'int'

Any ideas?