($) :: (Int -> Int) -> Int -> Int
-- pruning with 3/3 rules
-- looking through 1 candidates of size 1
-- looking through 1 candidates of size 2
-- tested 2 candidates
f $ x  =  f x

(.) :: (Int -> Int) -> (Int -> Int) -> Int -> Int
-- pruning with 3/3 rules
-- looking through 1 candidates of size 1
-- looking through 2 candidates of size 2
-- looking through 4 candidates of size 3
-- tested 5 candidates
(f . g) x  =  f (g x)

flip :: (Int -> Int -> Int) -> Int -> Int -> Int
-- pruning with 3/3 rules
-- looking through 2 candidates of size 1
-- looking through 0 candidates of size 2
-- looking through 4 candidates of size 3
-- tested 5 candidates
flip f x y  =  f y x

map :: (Int -> Int) -> [Int] -> [Int]
-- pruning with 3/3 rules
-- looking through 2 candidates of size 1
-- looking through 1 candidates of size 2
-- looking through 0 candidates of size 3
-- looking through 2 candidates of size 4
-- looking through 2 candidates of size 5
-- looking through 5 candidates of size 6
-- looking through 7 candidates of size 7
-- tested 13 candidates
map f []  =  []
map f (x:xs)  =  f x:map f xs

fold :: (Int -> Int -> Int) -> Int -> [Int] -> Int
-- pruning with 3/3 rules
-- looking through 1 candidates of size 1
-- looking through 1 candidates of size 2
-- looking through 1 candidates of size 3
-- looking through 6 candidates of size 4
-- looking through 4 candidates of size 5
-- looking through 23 candidates of size 6
-- looking through 19 candidates of size 7
-- tested 38 candidates
fold f x []  =  x
fold f x (y:xs)  =  fold f (f x y) xs

filter :: (Int -> Bool) -> [Int] -> [Int]
-- pruning with 3/3 rules
-- looking through 2 candidates of size 1
-- looking through 1 candidates of size 2
-- looking through 0 candidates of size 3
-- looking through 2 candidates of size 4
-- looking through 0 candidates of size 5
-- looking through 5 candidates of size 6
-- looking through 0 candidates of size 7
-- looking through 17 candidates of size 8
-- looking through 0 candidates of size 9
-- looking through 44 candidates of size 10
-- looking through 0 candidates of size 11
-- looking through 142 candidates of size 12
-- tested 72 candidates
filter f []  =  []
filter f (x:xs)  =  if f x
                    then x:filter f xs
                    else filter f xs