--- a/progs/app05.scala	Mon Oct 17 13:18:34 2016 +0100
+++ b/progs/app05.scala	Tue Oct 18 10:43:26 2016 +0100
@@ -3,8 +3,8 @@
     val (r1s, f1s) = simp(r1)
     val (r2s, f2s) = simp(r2)
     (r1s, r2s) match {
-      case (NULL, _) => (r2s, F_RIGHT(f2s))
-      case (_, NULL) => (r1s, F_LEFT(f1s))
+      case (ZERO, _) => (r2s, F_RIGHT(f2s))
+      case (_, ZERO) => (r1s, F_LEFT(f1s))
       case _ => 
          if (r1s == r2s) (r1s, F_LEFT(f1s))
          else (ALT (r1s, r2s), F_ALT(f1s, f2s))