1 def ders2(s: List[Char], r: Rexp) : Rexp = (s, r) match {

     2   case (Nil, r) => r

     3   case (s, ZERO) => ZERO

     4   case (s, ONE) => if (s == Nil) ONE else ZERO

     5   case (s, CHAR(c)) => if (s == List(c)) ONE else 

     6                        if (s == Nil) CHAR(c) else ZERO

     7   case (s, ALT(r1, r2)) => ALT(ders2(s, r2), ders2(s, r2))

     8   case (c::s, r) => ders2(s, der(c, r).simp)

     9 }