Right, but unless I miss my guess (I haven't looked at the code) it isn't that the concrete api is being used by the constructor, it's that the concrete API is being used by the iterator protocol called by the constructor.  If I'm wrong it would be sensible to fix the constructor and we can reopen this.  Even if I'm right perhaps there's a way to fix the constructor, but in that case it would be best addressed in the context of that issue.