From: "Marcin 'Qrczak' Kowalczyk" <qrczak@knm.org.pl>
| + can't be treated in the same way, because it won't distinguish
| whether it's called as 1 + 1 or true + true. If it returns 2 in the
| former case, it would produce a nonsensical bool value in the latter
| case.
| 
| OCaml doesn't have a mechanism for making +, - applicable to a limited
| set of types and for dispatching their implementation based on the type
| rather than on the physical representation.

This is an excellent point.  (Somebody else also pointed it out in a
reply to me.)

From: padiolea@irisa.fr
> In the module Obj of the caml library there is a function
>  external is_int : t -> bool = "%obj_is_int"

But it doesn't quite work for our purpose here, for example:

# type t = A | B ;;
type t = A | B
# Obj.is_int (Obj.magic A) ;;
- : bool = true

	Eijiro