stuffs

data Q elt = Construct [elt] [elt]

enqueue :: (Q a) -> a -> (Q a)
enqueue (Construct f b) el = Construct f (b : el)

dequeue :: (Q a) -> Maybe (a, Q a)
dequeue (Construct f b) =
 if length f > 0 then ((head f), Construct (tail f) b) else if length b == 0 then Nothing else ((head b_rev), Construct (tail b_rev) [])
 where
  b_rev = reverse b