eval :: Property -> (Bool -> IO a) -> (Pos -> IO a) -> IO a
eval p k u = answer p (\p -> eval' p k u) u
eval :: Property -> (Bool -> IO a) -> (Pos -> IO a) -> IO a
eval p k u = answer p (\p -> eval' p k u) u
eval p k u = answer p (\p -> eval' p k u) u