From f069c6a4576d9c4d26666371d08a2a85954c413e Mon Sep 17 00:00:00 2001
From: wadler <wadler@inf.ed.ac.uk>
Date: Tue, 27 Mar 2018 16:19:03 -0300
Subject: [PATCH] extra stuff

---
 src/extra/Exists.agda | 54 -------------------------------------------
 src/extra/Extra.lagda | 25 ++++++++++++++++++++
 2 files changed, 25 insertions(+), 54 deletions(-)
 delete mode 100644 src/extra/Exists.agda
 create mode 100644 src/extra/Extra.lagda

diff --git a/src/extra/Exists.agda b/src/extra/Exists.agda
deleted file mode 100644
index d3bbd2a2..00000000
--- a/src/extra/Exists.agda
+++ /dev/null
@@ -1,54 +0,0 @@
-import Relation.Binary.PropositionalEquality as Eq
-open Eq using (_≡_; refl; sym; trans; cong)
-open Eq.≡-Reasoning
-open import Data.Nat using (ℕ; zero; suc; _+_; _*_)
-open import Data.Nat.Properties.Simple using (+-suc; +-right-identity)
-open import Relation.Nullary using (¬_)
-open import Function using (_∘_)
-open import Data.Product using (_×_; _,_; proj₁; proj₂)
-open import Data.Sum using (_⊎_; inj₁; inj₂)
-
-data ∃ {A : Set} (B : A → Set) : Set where
-  _,_ : (x : A) → B x → ∃ B
-
-syntax ∃ {A} (λ x → B) = ∃[ x ∈ A ] B
-
-
-data even : ℕ → Set
-data odd  : ℕ → Set
-
-data even where
-  even-zero : even zero
-  even-suc  : ∀ {n : ℕ} → odd n → even (suc n)
-
-data odd where
-  odd-suc   : ∀ {n : ℕ} → even n → odd (suc n)
-
-lemma : ∀ (m : ℕ) →  2 * suc m ≡ suc (suc (2 * m))
-lemma m =
-  begin
-    2 * suc m
-  ≡⟨⟩
-    suc m + (suc m + zero)
-  ≡⟨⟩
-    suc (m + (suc (m + zero)))
-  ≡⟨ cong suc (+-suc m (m + zero)) ⟩
-    suc (suc (m + (m + zero)))
-  ≡⟨⟩
-    suc (suc (2 * m))
-  ∎
-
-∃-even : ∀ {n : ℕ} → even n → ∃[ m ∈ ℕ ] (n ≡ 2 * m)
-∃-odd  : ∀ {n : ℕ} →  odd n → ∃[ m ∈ ℕ ] (n ≡ 1 + 2 * m)
-
-∃-even even-zero =  zero , refl
-∃-even (even-suc o) with ∃-odd o
-...                    | m , eqn rewrite eqn = suc m , sym (lemma m)
-
-∃-odd  (odd-suc e)  with ∃-even e
-...                    | m , eqn rewrite eqn = m , refl
-
-∃-even′ : ∀ {n : ℕ} → even n → ∃[ m ∈ ℕ ] (n ≡ 2 * m)
-∃-even′ even-zero =  zero , refl
-∃-even′ (even-suc o) with ∃-odd o
-...                    | m , eqn rewrite eqn | cong suc (+-right-identity m) = suc m , {!!}
diff --git a/src/extra/Extra.lagda b/src/extra/Extra.lagda
new file mode 100644
index 00000000..1db33539
--- /dev/null
+++ b/src/extra/Extra.lagda
@@ -0,0 +1,25 @@
+<!--
+
+## Disjunction
+
+In order to state totality, we need a way to formalise disjunction,
+the notion that given two propositions either one or the other holds.
+In Agda, we do so by declaring a suitable inductive type.
+\begin{code}
+data _⊎_ : Set → Set → Set where
+  inj₁ : ∀ {A B : Set} → A → A ⊎ B
+  inj₂ : ∀ {A B : Set} → B → A ⊎ B
+\end{code}
+This tells us that if `A` and `B` are propositions then `A ⊎ B` is
+also a proposition.  Evidence that `A ⊎ B` holds is either of the form
+`inj₁ x`, where `x` is evidence that `A` holds, or `inj₂ y`, where
+`y` is evidence that `B` holds.
+
+We set the precedence of disjunction so that it binds less tightly than
+inequality.
+\begin{code}
+infixr 1 _⊎_
+\end{code}
+Thus, `m ≤ n ⊎ n ≤ m` parses as `(m ≤ n) ⊎ (n ≤ m)`.
+
+-->