diff --git a/tests/difference/core/quint_model/extraSpells.qnt b/tests/difference/core/quint_model/extraSpells.qnt
index a959db5251..cd2105beb7 100644
--- a/tests/difference/core/quint_model/extraSpells.qnt
+++ b/tests/difference/core/quint_model/extraSpells.qnt
@@ -1,4 +1,5 @@
 -*- mode: Bluespec; -*-
+// This module is just a library with utility functions (sometimes called spells in Quint).
 module extraSpells {
 
   pure def prepend(__list: List[a], __elem: a): List[a] = {