added Isabelle_System.components;

--- a/src/Pure/System/isabelle_system.scala	Tue Aug 04 19:20:24 2009 +0200
+++ b/src/Pure/System/isabelle_system.scala	Tue Aug 04 23:25:00 2009 +0200
@@ -298,6 +298,12 @@
 
   /** Isabelle resources **/
 
+  /* components */
+
+  def components(): List[String] =
+    getenv("ISABELLE_COMPONENTS").split(":").toList
+
+
   /* find logics */
 
   def find_logics(): List[String] =