#     type foo = ..
#         type foo += A | B of int
#           val is_a : foo -> bool = <fun>
#         type foo
#     Characters 13-21:
  type foo += A of int (* Error type is not open *)
              ^^^^^^^^
Error: Cannot extend type definition foo
#         type 'a foo = ..
#     Characters 1-30:
  type ('a, 'b) foo += A of int (* Error: type parameter mismatch *)
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: This extension does not match the definition of type foo
       They have different arities.
#                 module type S = sig type foo type foo += A of float end
#                 Characters 84-106:
    type foo += B of float (* Error foo does not have an extensible type *)
    ^^^^^^^^^^^^^^^^^^^^^^
Error: Type foo is not extensible
#         type foo = ..
#                     module M :
  sig
    type foo += A of int | B of string
    type foo += C of int | D of float
    
  end
#                     module type S =
  sig
    type foo += B of string | C of int
    type foo += D of float
    type foo += A of int
  end
#     module M_S : S
#         type 'a foo = ..
#         type _ foo += A : int -> int foo | B : int foo
#           val get_num : 'a foo -> 'a -> 'a option = <fun>
#         type 'a foo = .. constraint 'a = [> `Var ]
#     type 'a foo += A of 'a
#     Characters 11-12:
  let a = A 9 (* ERROR: Constraints not met *)
            ^
Error: This expression has type int but an expression was expected of type
         [> `Var ]
#     Characters 20-23:
  type 'a foo += B : int foo (* ERROR: Constraints not met *)
                     ^^^
Error: This type int should be an instance of type [> `Var ]
#         type foo = ..
#     module M : sig type foo += A of int end
#     val a1 : foo = M.A 10
#     module type S = sig type foo += private A of int end
#     module M_S : S
#           val is_s : foo -> bool = <fun>
#     Characters 10-18:
  let a2 = M_S.A 20 (* ERROR: Cannot create a value using a private constructor *)
           ^^^^^^^^
Error: Cannot create values of the private type foo
#         type foo = ..
#     module M : sig type foo += A1 of int end
#     type foo += A2 of int
#     type bar = ..
#     Characters 18-22:
  type bar += A3 = M.A1    (* Error: rebind wrong type *)
                   ^^^^
Error: The constructor M.A1 has type foo but was expected to be of type bar
#     module M : sig type foo += private B1 of int end
#     type foo += private B2 of int
#     Characters 18-22:
  type foo += B3 = M.B1  (* Error: rebind private extension *)
                   ^^^^
Error: The constructor M.B1 is private
#     Characters 13-24:
  type foo += C = Unknown  (* Error: unbound extension *)
              ^^^^^^^^^^^
Error: Unbound constructor Unknown
#                       module M : sig type foo type foo += A1 of int end
type M.foo += A2 of int
type 'a foo = ..
#     type 'a foo1 = 'a foo = ..
#     type 'a foo2 = 'a foo = ..
#           type 'a foo1 += A of int | B of 'a | C : int foo1
#           type 'a foo2 += D of int | E of 'a | F : int foo2
#         type +'a foo = ..
#     type 'a foo += A of (int -> 'a)
#     Characters 1-32:
  type 'a foo += B of ('a -> int) (* ERROR: Parameter variances are not satisfied *)
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: In this definition, expected parameter variances are not satisfied.
       The 1st type parameter was expected to be covariant,
       but it is injective contravariant.
#     Characters 1-40:
  type _ foo += C : ('a -> int) -> 'a foo (* ERROR: Parameter variances are not satisfied *)
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: In this definition, expected parameter variances are not satisfied.
       The 1st type parameter was expected to be covariant,
       but it is injective contravariant.
#     type 'a bar = ..
#     Characters 1-33:
  type +'a bar += D of (int -> 'a) (* ERROR: type variances do not match *)
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: This extension does not match the definition of type bar
       Their variances do not agree.
#                       module M : sig type exn += Foo of int * float | Bar : 'a list -> exn  end
#                   module M :
  sig exception Bar : 'a list -> exn exception Foo of int * float end
#     exception Foo of int * float
#     exception Bar : 'a list -> exn
#                   module M : sig type exn += Foo of int * float | Bar : 'a list -> exn  end
#         type foo = ..
#         type foo += Foo of int * int option | Bar of int option
#     val x : foo * foo = (Foo (3, Some 4), Bar (Some 5))
#     type foo += Foo of string
#     val y : foo * foo = (<extension>, Bar (Some 5))
#     exception Foo of int * int option
#     exception Bar of int option
#     val x : exn * exn = (Foo (3, Some 4), Bar (Some 5))
#     type foo += Foo of string
#     val y : exn * exn = (Foo (3, _), Bar (Some 5))
#         type foo = ..
#         type foo += Foo | Bar of int
#     val n1 : string = "Foo"
#     val n2 : string = "Bar"
#     val t : bool = true
#     val f : bool = false
#         val is_foo : 'a -> bool = <fun>
type foo += Foo
#     val f : bool = false
#     Exception: Invalid_argument "Obj.extension_name".
#     Exception: Invalid_argument "Obj.extension_id".
#