diff -r 1494ded298a6 -r 4e6064759aeb src/Pure/System/isabelle_system.scala
--- a/src/Pure/System/isabelle_system.scala	Thu Jun 18 15:02:53 2009 +0200
+++ b/src/Pure/System/isabelle_system.scala	Thu Jun 18 15:08:57 2009 +0200
@@ -114,6 +114,8 @@
     if (value != "") value else error("Undefined environment variable: " + name)
   }
 
+  override def toString = getenv("ISABELLE_HOME")
+
 
   /* file path specifications */