at least 2 worker threads to ensure some degree of lifeness, notably for asynchronous Document.print_state;

--- a/src/Pure/System/isabelle_process.ML	Mon Sep 19 14:40:38 2011 +0200
+++ b/src/Pure/System/isabelle_process.ML	Mon Sep 19 16:40:17 2011 +0200
@@ -177,6 +177,9 @@
 
     val _ = quick_and_dirty := true;
     val _ = Goal.parallel_proofs := 0;
+    val _ =
+      if Multithreading.max_threads_value () < 2
+      then Multithreading.max_threads := 2 else ();
     val _ = Context.set_thread_data NONE;
     val _ =
       Unsynchronized.change print_mode