Is there a policy which describes precisely what should go into a -devel subpackage?

Here is an example case where I'm unsure:

In OCaml there's a thing called the "toplevel". Think of it as being like the python command line (where you just type "python" on its own, then start typing in Python expressions).

$ ocaml -I +calendar Objective Caml version 3.09.3

# #load "unix.cma";;

# #load "str.cma";;

# #load "calendar.cma";;

# let d = Printer.DatePrinter.from_string "2007-01-01";;

val d : Printer.DatePrinter.t = <abstr> # Printer.DatePrinter.to_string (Date.next d `Week);;

- : string = "2007-01-08"

So my question: If I'm packaging ocaml-calendar (a library) then should the parts which make the above possible go into the main package or ocaml-calendar-devel?

Note that as far as I can see in Python -- and I'm not hugely familiar with that language so forgive me if I've made a mistake -- but it seems they go in the main package.

Another thing is that if I move the parts into the -devel subpackage, then ocaml-calendar will literally be empty!

Rich.