input: Write a function to calculate the maximum revenue from a list of prices.
  assert max_revenue([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]) == 9
  assert max_revenue([10, 20, 5, 25, 15, 30, 10]) == 25
  assert max_revenue([100, 90, 80, 70, 60, 50, 40]) == 0
output: def max_revenue(prices):
      max_rev = 0
      for i in range(1, len(prices)):
          rev = prices[i] - prices[i - 1]
          if rev > max_rev:
              max_rev = rev
      return max_rev