#         type 'b class_name = .. constraint 'b = < cast : 'a. 'a name -> 'a; .. >
and 'a name =
    Class : 'a class_name -> (< cast : 'a0. 'a0 name -> 'a0; .. > as 'a) name
#     exception Bad_cast
#           class type castable = object method cast : 'a name -> 'a end
#                 class type foo_t = object method cast : 'a name -> 'a method foo : string end
#     type 'b class_name += Foo : foo_t class_name
#                   class foo : foo_t
#                 class type bar_t =
  object
    method bar : string
    method cast : 'a name -> 'a
    method foo : string
  end
#     type 'b class_name += Bar : bar_t class_name
#                     class bar : bar_t
#         val clist : castable list ref = {contents = []}
#       val push_castable : #castable -> unit = <fun>
#               val pop_castable : unit -> castable = <fun>
#       - : unit = ()
# - : unit = ()
# - : unit = ()
#   val c1 : castable = <obj>
# val c2 : castable = <obj>
# val c3 : castable = <obj>
#       val f1 : foo = <obj>
# val f2 : foo = <obj>
# val f3 : foo = <obj>
#   Exception: Bad_cast.
# val b2 : bar = <obj>
# Exception: Bad_cast.
#