liftB2 :: (B.BDB -> a -> IO b) -> BDB -> a -> TCM b
liftB2 f x y = TCM $ f (unTCBDB x) y
liftB2 :: (B.BDB -> a -> IO b) -> BDB -> a -> TCM b
liftB2 f x y = TCM $ f (unTCBDB x) y
liftB2 f x y = TCM $ f (unTCBDB x) y