-- What is the type of foldMap . foldMap? Or foldMap . foldMap . foldMap, etc.? What do they do? 

foldMap :: Monoid m => (a -> m) -> t a -> m

foldMap . foldMap :: (Foldable t, Foldable t1, Monoid m) => (a -> m) -> t (t1 a) -> m
-- They fold for multi-level of foldables