diff --git a/src/HottBook/Chapter2.lagda.md b/src/HottBook/Chapter2.lagda.md
index 0cb18e8..7ced792 100644
--- a/src/HottBook/Chapter2.lagda.md
+++ b/src/HottBook/Chapter2.lagda.md
@@ -411,9 +411,10 @@ theorem2∙13∙1 m n = encode m n , equiv
         -- what = transport (λ n → code m n) (decode m n c) (r m)
         -- what = transport (λ n → code m n) (decode m n c) (r m)
         what = transport (λ n → code m n) (decode m n c) (r m)
-        res : what ≡ c
-        res = IH
-      in {!   !}
+
+        res : encode (suc m) (suc n) (ap suc (decode m n c)) ≡ c
+        res = {!   !}
+      in res
 
     equiv = record
       { g = decode m n