diff --git a/src/Data/Abstract/FreeVariables.hs b/src/Data/Abstract/FreeVariables.hs
index 770cc45ff..4412d34e8 100644
--- a/src/Data/Abstract/FreeVariables.hs
+++ b/src/Data/Abstract/FreeVariables.hs
@@ -12,6 +12,7 @@ import Data.Union
 type Name = ByteString
 
 
+-- | Types which can contain unbound variables.
 class FreeVariables term where
   freeVariables :: term -> Set Name