Example taken from the VOCaL project:
  let make (dummy [@ocaml:optional]: option 'a) (n: int63) (x: 'a) : t 'a
    ...

  let init (dummy [@ocaml:named]: 'a) (n: int63) (f: int63 -> 'a) : t 'a
  = let a = make None n dummy in
    ...

Now the resulting OCaml code is as follows:
  let init ~dummy:(dummy: 'a) (n: int) (f: (int) -> 'a) : 'a t =
    let a = make  n dummy in
    ...