--- a/progs/app03.scala	Wed Apr 06 11:51:33 2016 +0100
+++ b/progs/app03.scala	Wed Apr 06 12:18:54 2016 +0100
@@ -1,6 +1,6 @@
 abstract class Rexp
-case object NULL extends Rexp
-case object EMPTY extends Rexp
+case object ZERO extends Rexp
+case object ONE extends Rexp
 case class CHAR(c: Char) extends Rexp
 case class ALT(r1: Rexp, r2: Rexp) extends Rexp 
 case class SEQ(r1: Rexp, r2: Rexp) extends Rexp