Index: src/work/alpar/invalid.h
===================================================================
--- src/work/alpar/invalid.h	(revision 169)
+++ src/work/alpar/invalid.h	(revision 184)
@@ -16,5 +16,8 @@
   /// \ref Invalid is a global type that converts to each iterator
   /// in such a way that the value of the target iterator will be invalid.
-  
+
+  // It is also used to convert the \c INVALID constant to the
+  // node iterator that makes is possible to write 
+
   //extern Invalid INVALID;