input: plaintext
  from typing import List
  
  def max_revenue(prices: List[int]) -> int:
      """ Calculate the maximum revenue possible from a list of stock prices.
      The revenue is calculated by subtracting the price of a stock at one time
      from the price at a later time. This function finds the maximum positive
      difference possible in the list of prices.
      >>> max_revenue([10, 7, 5, 8, 11, 9])
      6
      """
      diff = [prices[i + 1] - prices[i] for i in range(len(prices) - 1)] 
      max_diff = 0
      for i in range(len(diff)): 
          for j in range(i + 1, len(diff) + 1): 
              current_diff = sum(diff[i:j]) 
              if current_diff > max_diff: 
                  max_diff = current_diff 
   
      return max_diff
  
output:     if not prices:
          return 0
      
      min_price = prices[0]
      max_profit = 0
      
      for price in prices:
          if price < min_price:
              min_price = price
          else:
              max_profit = max(max_profit, price - min_price)
      
      return max_profit