recursive modules: Cannot safely evaluate the definition

[Hendrik Tews <tews@cs.ru.nl>]

Hi,

for the following program

    module type Result_sig = sig
      val result : int
    end

    module type Build_sig = sig
      val compute : int -> unit
    end

    module rec Rec : functor(Arg : Result_sig) -> Build_sig = 
      functor(Arg : Result_sig) ->
    struct
      let compute = function
	| 0 -> Printf.printf "result: %d\n" Arg.result
	| n ->
	    let module Arg_x_2 = struct
	      let result = Arg.result * 2
	    end
	    in
	    let module Rec_tail = Rec(Arg_x_2)
	    in
	      Rec_tail.compute (n-1)
    end


I get the compiler error 

  Cannot safely evaluate the definition of the recursively-defined module Rec

Could somebody explain to me why? As far as I can see, module Rec
is safe in the sense of section 7.9 of the reference manual. 

[Background: I am trying to solve the following Ocaml puzzle:
Build a (nested) functor application, where the functors applied
depend on an input value of the program. In particular the depth
of the functor application depends on the input value and could
be arbitrary high.]


Bye,

Hendrik