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).
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.