diff --git a/src/lib/pickles/plonk_checks/plonk_checks.mli b/src/lib/pickles/plonk_checks/plonk_checks.mli
index 60df2dbe3240..365eb6690013 100644
--- a/src/lib/pickles/plonk_checks/plonk_checks.mli
+++ b/src/lib/pickles/plonk_checks/plonk_checks.mli
@@ -1,5 +1,6 @@
 open Pickles_types
 
+(* TODO: why do we use objects?*)
 type 'field vanishing_polynomial_domain =
   < vanishing_polynomial : 'field -> 'field >