fulfill_proof_future: tuned important special case of singleton promise;

--- a/src/Pure/proofterm.ML	Wed Nov 04 20:31:36 2009 +0100
+++ b/src/Pure/proofterm.ML	Wed Nov 04 21:21:05 2009 +0100
@@ -1296,6 +1296,8 @@
       in PBody {oracles = oracles, thms = thms, proof = proof} end;
 
 fun fulfill_proof_future _ [] body = Future.value body
+  | fulfill_proof_future thy [(i, promise)] body =
+      Future.map (fn p => fulfill_proof thy [(i, p)] body) promise
   | fulfill_proof_future thy promises body =
       Future.fork_deps (map snd promises) (fn () =>
         fulfill_proof thy (map (apsnd Future.join) promises) body);