diff -r 75623b4d6251 -r db3d3d99c69d src/Pure/Concurrent/future.ML
--- a/src/Pure/Concurrent/future.ML	Mon Nov 11 16:40:07 2013 +0100
+++ b/src/Pure/Concurrent/future.ML	Wed Nov 20 22:10:45 2013 +0100
@@ -48,8 +48,7 @@
   val worker_group: unit -> group option
   val the_worker_group: unit -> group
   val worker_subgroup: unit -> group
-  val worker_nest: string -> ('a -> 'b) -> 'a -> 'b
-  val worker_guest: string -> group -> ('a -> 'b) -> 'a -> 'b
+  val worker_context: string -> group -> ('a -> 'b) -> 'a -> 'b
   type 'a future
   val task_of: 'a future -> task
   val peek: 'a future -> 'a Exn.result option
@@ -110,13 +109,8 @@
 
 fun worker_subgroup () = new_group (worker_group ());
 
-fun worker_nest name f x =
-  setmp_worker_task (Task_Queue.new_task (the_worker_group ()) name NONE) f x;
-
-fun worker_guest name group f x =
-  if is_some (worker_task ()) then
-    raise Fail "Already running as worker thread"
-  else setmp_worker_task (Task_Queue.new_task group name NONE) f x;
+fun worker_context name group f x =
+  setmp_worker_task (Task_Queue.new_task group name NONE) f x;
 
 fun worker_joining e =
   (case worker_task () of