--jednoducha verze, stejna jako lookup z Data.List
lookUp :: Eq k => k -> [(k, v)] -> Maybe v
lookUp _ [] = Nothing
lookUp k ((l, v):r)
  | k == l = Just v
  | otherwise = lookUp k r

--4nasobny lookup
--(prvni nalezenou hodnotu pouzije jako klic pro druhe hledani, atd.)
lookUp4 :: Eq a => a -> [(a, a)] -> Maybe a
lookUp4 k s =
  case lookUp k s of
    Nothing -> Nothing
    Just s1 ->
      case lookUp s1 s of
        Nothing -> Nothing
        Just s2 ->
          case lookUp s2 s of
            Nothing -> Nothing
            Just s3 -> lookUp s3 s

--pomucka
andThen Nothing f = Nothing
andThen (Just a) f = f a

--trochu lepsi lookup4. `andThen` jde chapat jako strednik z Ccka.
lookUp4' k s =
  lookUp k s `andThen`
  \k -> lookUp k s `andThen`
  \k -> lookUp k s `andThen`
  \k -> lookUp k s