diff --git a/src/HottBook/Chapter2.lagda.md b/src/HottBook/Chapter2.lagda.md
index 2c2d9cf..ba73ffb 100644
--- a/src/HottBook/Chapter2.lagda.md
+++ b/src/HottBook/Chapter2.lagda.md
@@ -401,7 +401,9 @@ theorem2∙13∙1 m n = encode m n , equiv
         what4 = isequiv.g (Σ.snd what3)
         what5 = what4 tt
       in what5
-    backward (suc m) (suc n) c = {!   !}
+    backward (suc m) (suc n) c =
+      let
+      in {!   !}
 
     equiv = record
       { g = decode m n