abstract class Rexp+
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case object NULL extends Rexp+
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case object EMPTY extends Rexp+
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case class CHAR(c: Char) extends Rexp+
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case class ALT(r1: Rexp, r2: Rexp) extends Rexp +
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case class SEQ(r1: Rexp, r2: Rexp) extends Rexp +
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case class STAR(r: Rexp) extends Rexp +
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case class REC(x: String, r: Rexp) extends Rexp+
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