structure Report =

 struct

 local

     val report_depth = Unsynchronized.ref 0

     fun space n = if n <= 0 then "" else (space (n-1))^" "

     fun report_space () = space (!report_depth)

 in

 fun timeit f =

     let

         val t1 = Timing.start ()

         val x = f ()

         val t2 = Timing.message (Timing.result t1)

         val _ = writeln ((report_space ()) ^ "--> "^t2)

     in

         x       

     end

 fun report s f = 

 let

     val _ = writeln ((report_space ())^s)

     val _ = report_depth := !report_depth + 1

     val x = timeit f

     val _ = report_depth := !report_depth - 1

 in

     x

 end

 end

 end