genMaybe' :: Arbitrary a => Gen (Maybe a)
genMaybe' = do
  a <- arbitrary
  frequency [ (1, return Nothing)
            , (3, return $ Just a)]
genMaybe' :: Arbitrary a => Gen (Maybe a)
genMaybe' = do
  a <- arbitrary
  frequency [ (1, return Nothing)
            , (3, return $ Just a)]
genMaybe' = do
  a <- arbitrary
  frequency [ (1, return Nothing)
            , (3, return $ Just a)]