filterDbNumber :: [DatabaseItem] -> [Integer]
filterDbNumber = foldr f []
  where f (DbNumber x) xs = x : xs
        f _ xs = xs

-- 3.
filterDbNumber :: [DatabaseItem] -> [Integer]
filterDbNumber = foldr f []
  where f (DbNumber x) xs = x : xs
        f _ xs = xs

-- 3.
filterDbNumber = foldr f []
  where f (DbNumber x) xs = x : xs
        f _ xs = xs

-- 3.