diff -r b93f4d2aeee1 -r 36e5752fa191 progs/app06.scala
--- a/progs/app06.scala	Mon Oct 17 13:18:34 2016 +0100
+++ b/progs/app06.scala	Tue Oct 18 10:43:26 2016 +0100
@@ -3,10 +3,10 @@
     val (r1s, f1s) = simp(r1)
     val (r2s, f2s) = simp(r2)
     (r1s, r2s) match {
-      case (NULL, _) => (NULL, F_ERROR)
-      case (_, NULL) => (NULL, F_ERROR)
-      case (EMPTY, _) => (r2s, F_SEQ_Void1(f1s, f2s))
-      case (_, EMPTY) => (r1s, F_SEQ_Void2(f1s, f2s))
+      case (ZERO, _) => (ZERO, F_ERROR)
+      case (_, ZERO) => (ZERO, F_ERROR)
+      case (ONE, _) => (r2s, F_SEQ_Void1(f1s, f2s))
+      case (_, ONE) => (r1s, F_SEQ_Void2(f1s, f2s))
       case _ => (SEQ(r1s,r2s), F_SEQ(f1s, f2s))
     }
   }