max expr size  =    5
max  #-tests   =  500
min  #-tests   =   25  (to consider p ==> q true)
max  #-vars    =    3  (for inequational and conditional laws)

_ :: Bool  (holes: Bool)
_ :: Int  (holes: Int)
_ :: [Int]  (holes: [Int])
_ :: [(Int,Int)]  (holes: [(Int,Int)])
(++) :: [Int] -> [Int] -> [Int]
(==) :: Int -> Int -> Bool
length :: [Int] -> Int
zip :: [Int] -> [Int] -> [(Int,Int)]
True :: Bool
False :: Bool
(==) :: Bool -> Bool -> Bool
(==) :: [Int] -> [Int] -> Bool
(==) :: [(Int,Int)] -> [(Int,Int)] -> Bool

zip xs (xs ++ ys) == zip xs xs
zip (xs ++ ys) xs == zip xs xs

length xs == length ys ==> zip xs (ys ++ zs) == zip xs ys
length xs == length ys ==> zip (xs ++ zs) ys == zip xs ys