How do I instantiate a class as invariant?

[Craig Black]

I can't figure out how to instantiate a class as invariant.  The following 
code issues this error:  cannot implicitly convert expression (new A) of 
type hello.A to invariant(A)

class A {}

int main(char[][] args)
{
  invariant A a = new A;
  return 0;
}

-Craig