Martin Escardo, 29 June 2018

The subtype Ordinalsᵀ of ordinals with a top element.


{-# OPTIONS --safe --without-K #-}

open import UF.FunExt

module Ordinals.ToppedType
        (fe : FunExt)

open import MLTT.Spartan
open import Notation.CanonicalMap
open import Ordinals.Notions
open import Ordinals.Type
open import Ordinals.Underlying
open import UF.Base
open import UF.Sets
open import UF.Subsingletons


To get closure under sums constructively, we need to restrict to
particular kinds of ordinals. Having a top element is a simple
sufficient condition, which holds in the applications we have in mind
(for compact ordinals).  Classically, the topped ordinals are the
successor ordinals. Constructively, ℕ∞ is an example of an ordinal
with a top element which is not a successor ordinal, as its top
element is not isolated.


Ordinalᵀ :  𝓤  𝓤  ̇
Ordinalᵀ 𝓤 = Σ α  Ordinal 𝓤 , has-top (underlying-order α)

 canonical-map-Ordinalᵀ-Ordinal : Canonical-Map (Ordinalᵀ 𝓤) (Ordinal 𝓤)
 ι {{canonical-map-Ordinalᵀ-Ordinal}} (α , _) = α

 underlying-type-of-topped-ordinal : Underlying (Ordinalᵀ 𝓤)
 ⟨_⟩ {{underlying-type-of-topped-ordinal}} (α , _) =  α 
 underlying-order {{underlying-type-of-topped-ordinal}} (α , _) = underlying-order α

underlying-type-is-setᵀ : FunExt
                         (β : Ordinalᵀ 𝓤)
                         is-set  β 
underlying-type-is-setᵀ fe (α , t) = underlying-type-is-set fe α


Topped ordinals are ranged over by τ,υ.


tis-well-ordered : (τ : Ordinalᵀ 𝓤)  is-well-order (underlying-order τ)
tis-well-ordered ((X , _<_ , o) , t) = o

≾-prop-valued : (τ : Ordinalᵀ 𝓤) (x y :  τ )  is-prop (x ≾⟨ τ  y)
≾-prop-valued {𝓤} τ = ≾-is-prop-valued
                       (underlying-order τ)
                       (fe 𝓤 𝓤₀)
                       (Prop-valuedness [ τ ])

topped : (τ : Ordinalᵀ 𝓤)  has-top (underlying-order τ)
topped (α , t) = t

top : (τ : Ordinalᵀ 𝓤)   τ 
top (α , (x , i)) = x

top-is-top : (τ : Ordinalᵀ 𝓤)  is-top (underlying-order τ) (top τ)
top-is-top (α , (x , i)) = i

open import TypeTopology.InfProperty

has-infs-of-complemented-subsets : Ordinalᵀ 𝓤  𝓤 ̇
has-infs-of-complemented-subsets τ = has-inf (underlying-weak-order τ)