diff -r fb160aa7a9a8 -r 7f741b2c82a3 src/Pure/PIDE/document.scala
--- a/src/Pure/PIDE/document.scala	Sun Feb 26 18:19:44 2012 +0100
+++ b/src/Pure/PIDE/document.scala	Sun Feb 26 18:25:28 2012 +0100
@@ -307,7 +307,7 @@
       State().define_version(Version.init, Assignment.init).assign(Version.init.id)._2
   }
 
-  sealed case class State(
+  sealed case class State private(
     val versions: Map[Version_ID, Version] = Map.empty,
     val commands: Map[Command_ID, Command.State] = Map.empty,  // static markup from define_command
     val execs: Map[Exec_ID, Command.State] = Map.empty,  // dynamic markup from execution