deRefWeak, -- :: Weak v -> IO (Maybe v)
-- finalise -- :: Weak v -> IO ()
-- replaceFinaliser -- :: Weak v -> IO () -> IO ()
+ mkWeakNoFinaliser, -- :: k -> v -> IO (Weak v)
mkWeakPtr, -- :: k -> IO () -> IO (Weak k)
mkWeakPair, -- :: k -> v -> IO () -> IO (Weak (k,v))