Bottom.lagda.md

<!--
```agda
open import Cat.Diagram.Initial
open import Cat.Prelude

open import Order.Diagram.Lub
open import Order.Base
open import Order.Cat

import Order.Reasoning
```
-->

```agda
module Order.Diagram.Bottom {o ℓ} (P : Poset o ℓ) where
```

<!--
```agda
open Order.Reasoning P

open is-lub
open Lub
```
-->

# Bottom elements {defines="bottom-element"}

A **bottom element** in a partial order $(P, \le)$ is an element $\bot :
P$ that is smaller than any other element of $P$. This is the same as
being a least upper upper bound for the empty family $\bot \to P$.

```agda
is-bottom : Ob → Type _
is-bottom bot = ∀ x → bot ≤ x

record Bottom : Type (o ⊔ ℓ) where
  no-eta-equality
  field
    bot : Ob
    has-bottom : is-bottom bot

  ¡ : ∀ {x} → bot ≤ x
  ¡ = has-bottom _

is-bottom→is-lub : ∀ {lub} {f : ⊥ → _} → is-bottom lub → is-lub P f lub
is-bottom→is-lub is-bot .least x _ = is-bot x

is-lub→is-bottom : ∀ {lub} {f : ⊥ → _} → is-lub P f lub → is-bottom lub
is-lub→is-bottom lub x = lub .least x λ ()
```

<!--
```agda
is-bottom-is-prop : ∀ x → is-prop (is-bottom x)
is-bottom-is-prop _ = hlevel 1

bottom-unique : ∀ {x y} → is-bottom x → is-bottom y → x ≡ y
bottom-unique p q = ≤-antisym (p _) (q _)

Bottom-is-prop : is-prop Bottom
Bottom-is-prop p q i .Bottom.bot =
  bottom-unique (Bottom.has-bottom p) (Bottom.has-bottom q) i
Bottom-is-prop p q i .Bottom.has-bottom =
  is-prop→pathp
    (λ i → is-bottom-is-prop (bottom-unique (Bottom.has-bottom p) (Bottom.has-bottom q) i))
    (Bottom.has-bottom p) (Bottom.has-bottom q) i

instance
  H-Level-Bottom
    : ∀ {n}
    → H-Level Bottom (suc n)
  H-Level-Bottom = prop-instance Bottom-is-prop

Bottom→Lub : ∀ {f : ⊥ → _} → Bottom → Lub P f
Bottom→Lub bottom .Lub.lub = Bottom.bot bottom
Bottom→Lub bottom .Lub.has-lub = is-bottom→is-lub (Bottom.has-bottom bottom)

Lub→Bottom : ∀ {f : ⊥ → _} → Lub P f → Bottom
Lub→Bottom lub .Bottom.bot = Lub.lub lub
Lub→Bottom lub .Bottom.has-bottom = is-lub→is-bottom (Lub.has-lub lub)

is-bottom≃is-lub : ∀ {lub} {f} → is-equiv (is-bottom→is-lub {lub} {f})
is-bottom≃is-lub = biimp-is-equiv! _ is-lub→is-bottom

Bottom≃Lub : ∀ {f} → is-equiv (Bottom→Lub {f})
Bottom≃Lub = biimp-is-equiv! _ Lub→Bottom
```
-->

## As initial objects

Bottoms are the decategorifcation of [[initial objects]]; we don't need to
require the uniqueness of the universal morphism, as we've replaced our
hom-sets with hom-props!

```agda
is-bottom→initial : ∀ {x} → is-bottom x → is-initial (poset→category P) x
is-bottom→initial is-bot x .centre = is-bot x
is-bottom→initial is-bot x .paths _ = ≤-thin _ _

initial→is-bottom : ∀ {x} → is-initial (poset→category P) x → is-bottom x
initial→is-bottom initial x = initial x .centre
```