--- a/progs/app52.scala	Mon Oct 03 00:24:45 2016 +0100
+++ b/progs/app52.scala	Mon Oct 03 00:37:28 2016 +0100
@@ -5,5 +5,5 @@
   case (s, CHAR(c)) => if (s == List(c)) ONE else 
                        if (s == Nil) CHAR(c) else ZERO
   case (s, ALT(r1, r2)) => ALT(ders2(s, r2), ders2(s, r2))
-  case (c::s, r) => ders2(s, der(c, r).simp)
+  case (c::s, r) => ders2(s, simp(der(c, r)))
 }