diff --git a/src/HottBook/Chapter2.lagda.md b/src/HottBook/Chapter2.lagda.md
index e1e43ed..841d9dc 100644
--- a/src/HottBook/Chapter2.lagda.md
+++ b/src/HottBook/Chapter2.lagda.md
@@ -404,7 +404,10 @@ theorem2∙13∙1 m n = encode m n , equiv
     backward (suc m) (suc n) c =
       let
         IH = backward m n c
-        what = encode (suc m) (suc n) (decode (suc m) (suc n) c)
+        what : code m n
+        what = encode (suc m) (suc n) (ap suc (decode m n c))
+        res : what ≡ c
+        res = {!   !}
       in {!   !}
 
     equiv = record