Parameter Args_to_Args2.X

  step ~f:(fun f x -> f ~foo:x) : ('a -> 'r1, 'r2) t -> (foo:'a -> 'r1, 'r2) t

The preferred way to factor out an Args sub-sequence:

  let args =
    Foo.Args.(
      bar "A"
      (* TODO: factor out the common baz qux sub-sequence *)
      @> baz "B"
      @> qux "C"
      @> zap "D"
      @> nil
    )

is to write a function that prepends the sub-sequence:

  let baz_qux remaining_args =
    Foo.Args.(
      baz "B"
      @> qux "C"
      @> remaining_args
    )

and splice it back into the original sequence using @@ so that things line up nicely:

  let args =
    Foo.Args.(
      bar "A"
      @> baz_qux
      @@ zap "D"
      @> nil
    )