The yield statement cannot be used inside an anonymous method or lambda expression

I thought I was being pretty cool with this code:

IEnumerator System.Collections.Generic.IEnumerable.GetEnumerator( ) {
  Action<UnbalancedBinaryTree> yielder = null;
  yielder = t => {
    yielder(t.Left);
    if (!t.IsEmpty)
      yield return t.Value;
    yielder(t.Right);
  };
  yielder(this);
}

But as it turns out (and I think I knew before, but had forgotten), C# doesn’t accept this. Not in 3.0 at least, and I think not in 4.0 either. Need to test. Meanwhile — why? Hm…