prop_heads xs n = n > 0 ==>
 runner1 (enumSpecial xs n $ heads xs) == P.length xs
  where types = xs :: [Int]
prop_heads xs n = n > 0 ==>
 runner1 (enumSpecial xs n $ heads xs) == P.length xs
  where types = xs :: [Int]
prop_heads xs n = n > 0 ==>
 runner1 (enumSpecial xs n $ heads xs) == P.length xs
  where types = xs :: [Int]