isHidden (x:xs) ys
  | elem x ys = x : isHidden xs ys
  | otherwise = isHidden xs ys
isHidden (x:xs) ys
  | elem x ys = x : isHidden xs ys
  | otherwise = isHidden xs ys
isHidden (x:xs) ys
  | elem x ys = x : isHidden xs ys
  | otherwise = isHidden xs ys