hashes of hashes not supported

[Dan]

Is there a fundamental problem with this?
The second assert causes compile below. Without the postblit 
there is no error.

Thanks
Dan

Error: function postblit.S.__postblit () is not callable using 
argument types () const
//postblit.d(6): Error: template instance 
object.AssociativeArray!(string,const(S)) error instantiating

------------------------------------
struct S {
   this(this) {}
}

void main() {
   S[string] s1, s2;
   assert(s1==s2);

   S[S[string]] w1, w2;
   assert(w1==w2);
}