The `[@@blah]` syntax refers to the extension-points feature that was introduced in Ocaml 4.02: https://whitequark.org/blog/2014/04/16/a-guide-to-extension-poi

The `[@@blah]` syntax refers to the extension-points feature that was introduced in Ocaml 4.02: https://whitequark.org/blog/2014/04/16/a-guide-to-extension-points-in-ocaml/ https://whitequark.org/blog/2014/04/16/a-guide-to-extension-... https://blog.janestreet.com/extension-points-or-how-ocaml-is-becoming-more-like-lisp/ https://blog.janestreet.com/extension-points-or-how-ocaml-is... These can be used to do the sort of metaprogramming that people used to do with camlp4. If you want to search more, the other keyword for googling would be "ppx".