diff --git a/src/HottBook/Chapter2.lagda.md b/src/HottBook/Chapter2.lagda.md
index 8a8faaa..e1e43ed 100644
--- a/src/HottBook/Chapter2.lagda.md
+++ b/src/HottBook/Chapter2.lagda.md
@@ -404,6 +404,7 @@ 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)
       in {!   !}
 
     equiv = record