Used for making an Identifiable module. Here's an example.
module Id = struct
module T = struct
type t = A | B [@@deriving bin_io, compare, sexp]
let hash (t : t) = Hashtbl.hash t
include Sexpable.To_stringable (struct type nonrec t = t [@@deriving sexp] end)
end
include T
include Identifiable.Make (T)
end