deniok

instance (Traversable t) => Foldable t where
  foldMap = Data.Traversable.foldMapDefault

instance (OtherFoldableChild t) => Foldable t where
  foldMap = Data.OtherFoldableChild.foldMapDefaultOther

f :: (Traversable t, OtherFoldableChild t) => t a -> ...
f xs = ... (foldr (+) 0 xs) ...