diff --git a/src/HottBook/Chapter2.lagda.md b/src/HottBook/Chapter2.lagda.md
index 8da10c6..204c268 100644
--- a/src/HottBook/Chapter2.lagda.md
+++ b/src/HottBook/Chapter2.lagda.md
@@ -380,5 +380,5 @@ theorem2∙13∙1 m n = {!   !}
 
     decode : (m n : ℕ) → code m n → m ≡ n
     decode zero zero c = refl
-    decode (suc m) (suc n) c = {!   !}
+    decode (suc m) (suc n) c = ap suc (decode m n c)
 ```
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