Please tell me how to use Core.

Install core in opam as follows on osx.

opam install core

Write the following program ex01.ml and

open Core.Std

let_=
  let list = [1;2;3] in
  letrc=List.fold_leftlist~init:0~f:(fun accx->
    acc+x
  ) in
 Printf.printf "%d\n"rc

ex01.ml

Compiled with the following command:

$ocamlfindocamlopt-package core ex01.ml

The following error appears:

ocamlfind: Error from package `threads': Missing-thread or-vmthread switch

It is said that the threads package switch is not working, but I am wondering what threads are.

ocamlbuild-use-ocamlfind-package core-tag thread ex01.native

If I wrote it like this, it worked, but I still don't know when I used omake.
It would be great if you could understand how to write using ocamlfind.