diff --git a/lib/default.nix b/lib/default.nix
index 09a64f754d8f..632b12510e82 100644
--- a/lib/default.nix
+++ b/lib/default.nix
@@ -1,3 +1,8 @@
+/* Library of low-level helper functions for nix expressions.
+ *
+ * Please implement (mostly) exhaustive unit tests
+ * for new functions in `./tests.nix'.
+ */
 let
 
   # trivial, often used functions