Fix #488

src/plfa/part2/Lambda.lagda.md

@@ -53,12 +53,13 @@ four.
 ## Imports
 
 ```
-open import Relation.Binary.PropositionalEquality using (_≡_; _≢_; refl)
-open import Data.String using (String; _≟_)
-open import Data.Nat using (ℕ; zero; suc)
 open import Data.Empty using (⊥; ⊥-elim)
-open import Relation.Nullary using (Dec; yes; no; ¬_)
 open import Data.List using (List; _∷_; [])
+open import Data.Nat using (ℕ; zero; suc)
+open import Data.Product using (∃-syntax; _×_)
+open import Data.String using (String; _≟_)
+open import Relation.Nullary using (Dec; yes; no; ¬_)
+open import Relation.Binary.PropositionalEquality using (_≡_; _≢_; refl)
 ```
 
 ## Syntax of terms
@@ -788,24 +789,30 @@ while if the top two lines stand for a single reduction
 step and the bottom two stand for zero or more reduction
 steps it is called the diamond property. In symbols:
 
-    confluence : ∀ {L M N} → ∃[ P ]
-      ( ((L —↠ M) × (L —↠ N))
+```
+postulate
+  confluence : ∀ {L M N}
+    → ((L —↠ M) × (L —↠ N))
       --------------------
-      → ((M —↠ P) × (N —↠ P)) )
+    → ∃[ P ] ((M —↠ P) × (N —↠ P))
 
-    diamond : ∀ {L M N} → ∃[ P ]
-      ( ((L —→ M) × (L —→ N))
+  diamond : ∀ {L M N}
+    → ((L —→ M) × (L —→ N))
       --------------------
-      → ((M —↠ P) × (N —↠ P)) )
+    → ∃[ P ] ((M —↠ P) × (N —↠ P))
+```
 
 The reduction system studied in this chapter is deterministic.
 In symbols:
 
+```
+postulate
   deterministic : ∀ {L M N}
     → L —→ M
     → L —→ N
       ------
     → M ≡ N
+```
 
 It is easy to show that every deterministic relation satisfies
 the diamond property, and that every relation that satisfies