diff --git a/semantic-tags/src/AST/Element.hs b/semantic-tags/src/AST/Element.hs
index 2fae2c913..f03b277ce 100644
--- a/semantic-tags/src/AST/Element.hs
+++ b/semantic-tags/src/AST/Element.hs
@@ -45,6 +45,7 @@ type family (a :: Side) <> (b :: Side) :: Side where
 class Element' (side :: Side) sub sup where
   prj' :: sup a -> Maybe (sub a)
 
+-- | Membership is reflexive.
 instance Element' 'Here t t where
   prj' = Just