src/Pure/System/isabelle_system.scala

--- a/src/Pure/System/isabelle_system.scala	Thu Jun 04 22:08:20 2009 +0200
+++ b/src/Pure/System/isabelle_system.scala	Thu Jun 04 22:52:53 2009 +0200
@@ -17,6 +17,12 @@
   val charset = "UTF-8"
 
 
+  /* unique ids */
+
+  private var id_count: BigInt = 0
+  def id(): String = synchronized { id_count += 1; "j" + id_count }
+
+
   /* Isabelle environment settings */
 
   private val environment = System.getenv