diff --git a/semantic-tags/src/AST/Element.hs b/semantic-tags/src/AST/Element.hs
index f03b277ce..ee1cb3ca1 100644
--- a/semantic-tags/src/AST/Element.hs
+++ b/semantic-tags/src/AST/Element.hs
@@ -49,6 +49,7 @@ class Element' (side :: Side) sub sup where
 instance Element' 'Here t t where
   prj' = Just
 
+-- | Membershp on the left.
 instance Element     t l
       => Element' 'L t (l :+: r) where
   prj' (L1 l) = prj l