(** Least upper bounds. [lub x y] is specified only when [x] and [y] have a common supertype; it can be arbitrary otherwise. *)
Parameter lub: t -> t -> t.

(* Specifications: *)
Axiom lub_left: forall x y z, sub x z -> sub y z -> sub x (lub x y).
Axiom lub_right: forall x y z, sub x z -> sub y z -> sub y (lub x y).
Axiom lub_min: forall x y z, sub x z -> sub y z -> sub (lub x y) z.
Parameter glb: t -> t -> t.
(** Greater lower bounds. [glb x y] is specified only when [x] and [y] have a common subtype; it can be arbitrary otherwise.*)
Axiom glb_left: forall x y z, sub z x -> sub z y -> sub (glb x y) x.
Axiom glb_right: forall x y z, sub z x -> sub z y -> sub (glb x y) y.
Axiom glb_max: forall x y z, sub z x -> sub z y -> sub z (glb x y).
Parameter low_bound: t -> t.