-- | @(lengthIs xs n) = (length xs == n)@
lengthIs :: [a] -> Int -> Bool
lengthIs lst n
  | n < 0
  = False
  | otherwise
  = atLength null False lst n
lengthIs :: [a] -> Int -> Bool
lengthIs lst n
  | n < 0
  = False
  | otherwise
  = atLength null False lst n
lengthIs lst n
  | n < 0
  = False
  | otherwise
  = atLength null False lst n