diff -r 4b2496bc79b2 -r 3b1136fb6bee progs/app06.scala
--- a/progs/app06.scala	Wed Oct 21 16:09:44 2020 +0100
+++ b/progs/app06.scala	Sat Oct 24 13:02:18 2020 +0100
@@ -5,17 +5,16 @@
     (r1s, r2s) match {
       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 (ONE, _) => (r2s, F_SEQ_Empty1(f1s, f2s))
+      case (_, ONE) => (r1s, F_SEQ_Empty2(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_Empty1(f1: Val => Val, f2: Val => Val) = 
+  (v:Val) => Sequ(f1(Empty), f2(v))
+def F_SEQ_Empty2(f1: Val => Val, f2: Val => Val) = 
+  (v:Val) => Sequ(f1(v), f2(Empty))
 def F_SEQ(f1: Val => Val, f2: Val => Val) = 
   (v:Val) => v match {
     case Sequ(v1, v2) => Sequ(f1(v1), f2(v2)) }