data Queue a = Queue [a] [a]  -- front & back lists

insert :: a -> Queue a -> Queue a
insert x (Queue f b) = Queue f (x:b)

pop :: Queue a -> (Maybe a, Queue a)
pop (Queue [] [])    = (Nothing, Queue [] [])
pop (Queue (x:xs) b) = (Just x, Queue xs b)
pop (Queue [] b)     = pop $ Queue (reverse b) []