diff -r e7d0157f0471 -r 16742bf62365 progs/app06.scala
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/progs/app06.scala	Sat Oct 08 16:44:11 2016 +0100
@@ -0,0 +1,21 @@
+def simp(r: Rexp): (Rexp, Val => Val) = r match {
+  case SEQ(r1, r2) => {
+    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 _ => (SEQ(r1s,r2s), F_SEQ(f1s, f2s))
+    }
+  }
+  ...
+
+def F_SEQ_Void1(f1: Val => Val, f2: Val => Val) = 
+  (v:Val) => Sequ(f1(Void), f2(v))
+def F_SEQ_Void2(f1: Val => Val, f2: Val => Val) = 
+  (v:Val) => Sequ(f1(v), f2(Void))
+def F_SEQ(f1: Val => Val, f2: Val => Val) = 
+  (v:Val) => v match {
+    case Sequ(v1, v2) => Sequ(f1(v1), f2(v2)) }