Index: index.html
===================================================================
--- index.html	(revision 5170)
+++ index.html	(revision 5171)
@@ -12,7 +12,7 @@
 	<li> <a href="01_intro/index.html">   1. Introduction
 	<li> <a href="02_model/index.html">   2. Model of the world
 	<li> <a href="03_data/">    3. Where data is stored
-	<li> <a href="04_ui/">      4. User interface
+	<li> <a href="04_ui/index.html">      4. User interface
 	<li> <a href="05_invoc/">   5. Program invocation
 	<li> <a href="06_feature/"> 6. Feature plugins
 	<li> <a href="07_io/">      7. File formats