diff --git a/src/trees.lagda.md b/src/trees.lagda.md
index b60ff661e3..a2f7fd445b 100644
--- a/src/trees.lagda.md
+++ b/src/trees.lagda.md
@@ -4,6 +4,8 @@
 {-# OPTIONS --guardedness #-}
 ```
 
+## Files in the `trees` module
+
 ```agda
 module trees where