{- Exercise 13.1 -}
data Formula a = Atom a | Con (Formula a) (Formula a) | Dis (Formula a) (Formula a) | Neg (Formula a) | Top | Bot
  deriving (Show, Eq)

simp :: Formula a -> Formula a
simp f = undefined

substitute :: Eq a => Formula a -> a -> Bool -> Formula a
substitute f a val = undefined


{- Exercise 13.2 -}
-- use foldr
prefixes :: [a] -> [[a]]
prefixes = undefined

-- use foldl
suffixes :: [a] -> [[a]]
suffixes = undefined


{- Exercises 13.3 -}
firstMissing :: [Integer] -> Integer
firstMissing = undefined


{- Exercise 13.4 -}
data Tree a = Empty | Node (Tree a) a (Tree a)

treeHeight :: Tree a -> Integer
treeHeight = undefined