 d :=   match d with   | Nil => D0 Nil   | D0 d => D0 d   | D0 d => D0 d   | D0 d => D0 d   | D0 d => D0 d   | D0 d => D0 d   | D0 d => D0 d   | D0 d => D0 d   | D0 d => D0 d   | D0 d => D0 d   | D0 d => D