+mapAccumBagLM :: Monad m
+ => (acc -> x -> m (acc, y)) -- ^ combining funcction
+ -> acc -- ^ initial state
+ -> Bag x -- ^ inputs
+ -> m (acc, Bag y) -- ^ final state, outputs
+mapAccumBagLM _ s EmptyBag = return (s, EmptyBag)
+mapAccumBagLM f s (UnitBag x) = do { (s1, x1) <- f s x; return (s1, UnitBag x1) }
+mapAccumBagLM f s (TwoBags b1 b2) = do { (s1, b1') <- mapAccumBagLM f s b1
+ ; (s2, b2') <- mapAccumBagLM f s1 b2
+ ; return (s2, TwoBags b1' b2') }
+mapAccumBagLM f s (ListBag xs) = do { (s', xs') <- mapAccumLM f s xs
+ ; return (s', ListBag xs') }
+