module Callback: Callback;
let register: (string, 'a) => unit;
Callback.register n v registers the value v under
the name n. C code can later retrieve a handle to v
by calling caml_named_value(n).
let register_exception: (string, exn) => unit;
Callback.register_exception n exn registers the
exception contained in the exception value exn
under the name n. C code can later retrieve a handle to
the exception by calling caml_named_value(n). The exception
value thus obtained is suitable for passing as first argument
to raise_constant or raise_with_arg.