winner :: Board -> Maybe Player
winner b | win b Xer = Just Xer
         | win b Oer = Just Oer
         | otherwise = Nothing
winner :: Board -> Maybe Player
winner b | win b Xer = Just Xer
         | win b Oer = Just Oer
         | otherwise = Nothing
winner b | win b Xer = Just Xer
         | win b Oer = Just Oer
         | otherwise = Nothing