src/Pure/library.ML

   val exists: ('a -> bool) -> 'a list -> bool
   val forall: ('a -> bool) -> 'a list -> bool
   val set: bool ref -> bool
   val reset: bool ref -> bool
   val toggle: bool ref -> bool
+  val change: 'a ref -> ('a -> 'a) -> unit
   val setmp: 'a ref -> 'a -> ('b -> 'c) -> 'b -> 'c
 
   (*lists*)
   exception LIST of string
   val null: 'a list -> bool

 
 fun set flag = (flag := true; true);
 fun reset flag = (flag := false; false);
 fun toggle flag = (flag := not (! flag); ! flag);
 
+fun change r f = r := f (! r);
+
 (*temporarily set flag, handling errors*)
 fun setmp flag value f x =
   let
     val orig_value = ! flag;
     fun return y = (flag := orig_value; y);

 fun untabify chs =
   let
     val tab_width = 8;
 
     fun untab (_, "\n") = (0, ["\n"])
-      | untab (pos, "\t") = let val d = tab_width - (pos mod tab_width) in (pos + d, replicate d " ") end
+      | untab (pos, "\t") =
+          let val d = tab_width - (pos mod tab_width) in (pos + d, replicate d " ") end
       | untab (pos, c) = (pos + 1, [c]);
   in
     if not (exists (equal "\t") chs) then chs
     else flat (#2 (foldl_map untab (0, chs)))
   end;