def rev(r: Rexp) : Rexp = r match {+
−
  case ZERO => ZERO+
−
  case ONE => ONE+
−
  case CHAR(c) => CHAR(c)+
−
  case ALT(r1, r2) => ALT(rev(r1), rev(r2))+
−
  case SEQ(r1, r2) => SEQ(rev(r2), rev(r1))+
−
  case STAR(r) => STAR(rev(r))+
−
}+
−