execute document version at high priority;

--- a/src/Pure/Isar/isar_document.ML	Tue Jul 20 18:33:19 2010 +0200
+++ b/src/Pure/Isar/isar_document.ML	Tue Jul 20 20:10:27 2010 +0200
@@ -241,7 +241,7 @@
     let
       val old_execution = ! execution;
       val _ = Future.cancel old_execution;
-      val new_execution = Future.fork (fn () =>
+      val new_execution = Future.fork_pri 1 (fn () =>
        (uninterruptible (K Future.join_result) old_execution;
         fold_entries no_id (fn (_, {state, ...}) => fn () => force_state state) document ()));
     in execution := new_execution end);