corebuild "Compilation successful" ... but where's the binary?

Hi,

In the RWO book, at the end of Chapter 1 under "Compiling and Running"
it says to run the corebuild command to compile a program from source
(sum.native). But this doesn't seem to be creating a binary...

    % cat sum.native
    open Core.Std
    let rec read_and_accumulate accum =
      let line = In_channel.input_line In_channel.stdin in
      match line with
      | None -> accum
      | Some x -> read_and_accumulate (accum +. Float.of_string x)
    let () =
      printf "Total: %F\n" (read_and_accumulate 0.)

    % corebuild sum.native
    Finished, 0 targets (0 cached) in 00:00:00.

The resulting directory structure after `corebuild sum.native` :

    % tree /home/john/example/sum
    /home/john/example/sum
    ├── _build
    │   ├── _digests
    │   ├── _log
    │   └── sum.native
    └── sum.native -> /home/john/example/sum/_build/sum.native

The log file says compilation was successful:

    % cat _build/_log
    ### Starting build.
    # Compilation successful.

But I'm not seeing a compiled binary...is there supposed to be one?
(the _build/sum.native in there is identical to the original source)

I *think* everything required is installed & up-to-date:

    % corebuild -version
    ocamlbuild 4.02.1

    % opam info core
                package: core
                version: 112.24.01
                repository: default

    % ocaml -version
    The OCaml toplevel, version 4.02.1

Thanks for any clues/clarification,

John

-- 
John Magolske
http://B79.net/contact