broken queue instance for fingertree.

hs/DataStructures.hs

        Deep p (append (Node3 a b c) t) (Two d x)
    append x (Deep p t s) = Deep p t (append x s)
instance Queue FingerTree where
    enqueue x ft = append x ft
    dequeue Empty = Nothing
    dequeue (Single x) = Just (x, Empty)
    dequeue ft =
        case last ft of
            Just x -> Just (x, rest ft) -- broken, we'd need a different version of rest
-- examples
sl = fromList [1..10] :: List Integer
bq = fromList [1..10] :: BankersQueue Integer