1 abstract class Rexp

     2 case object NULL extends Rexp

     3 case object EMPTY extends Rexp

     4 case class CHAR(c: Char) extends Rexp

     5 case class ALT(r1: Rexp, r2: Rexp) extends Rexp 

     6 case class SEQ(r1: Rexp, r2: Rexp) extends Rexp 

     7 case class STAR(r: Rexp) extends Rexp