+foldlBag :: (r -> a -> r) -> r
+ -> Bag a
+ -> r
+
+foldlBag k z EmptyBag = z
+foldlBag k z (UnitBag x) = k z x
+foldlBag k z (TwoBags b1 b2) = foldlBag k (foldlBag k z b1) b2
+foldlBag k z (ListBag xs) = foldl k z xs
+foldlBag k z (ListOfBags bs) = foldl (\r b -> foldlBag k r b) z bs
+