[reason] Private row types http://caml.inria.fr/pub/docs/manual-ocaml/extn.html

PrivateRowTypes.re

module F = (X: {type c = pri {.. x: int};}) => {
  let get_x = (o: X.c) => o#x;
};
module G =
       (
         X: {
           type c =
             pri {
               ..
               x: int,
               y: int,
             };
         },
       ) => {
  include F(X);
  let get_y = (o: X.c) => o#y;
};
type t = [ | `A(int) | `B(bool)];
type u = pri [> t];
type v = pri [< t > `A];