src/Pure/Isar/isar_document.ML

 fun execute document =
   NAMED_CRITICAL "Isar" (fn () =>
     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);
 
 in