Hilbert_Classical: disable multithreading altogether, otherwise proof normalization will fork futures independently of Goal.parallel_proofs;

--- a/src/HOL/ex/ROOT.ML	Wed Jun 02 11:09:26 2010 +0200
+++ b/src/HOL/ex/ROOT.ML	Wed Jun 02 13:18:21 2010 +0200
@@ -70,7 +70,7 @@
 HTML.with_charset "utf-8" (no_document use_thys)
   ["Hebrew", "Chinese", "Serbian"];
 
-(setmp_noncritical proofs 2 (setmp_noncritical Goal.parallel_proofs 0 use_thy))
+(setmp_noncritical proofs 2 (setmp_noncritical Multithreading.max_threads 1 use_thy))
   "Hilbert_Classical";
 
 use_thy "SVC_Oracle";