def der (r: Rexp, c: Char) : Rexp = r match {
  case NULL => NULL
  case EMPTY => NULL
  case CHAR(d) => if (c == d) EMPTY else NULL
  case ALT(r1, r2) => ALT(der(r1, c), der(r2, c))
  case SEQ(r1, r2) => 
    if (nullable(r1)) ALT(SEQ(der(r1, c), r2), der(r2, c))
    else SEQ(der(r1, c), r2)
  case STAR(r) => SEQ(der(r, c), STAR(r))
}

def ders (s: List[Char], r: Rexp) : Rexp = s match {
  case Nil => r
  case c::s => ders(s, der(c, r))
}