input: Write a function to generate a list of all substrings of a given string, sorted in order from longest to shortest length.
  def substring_in_order(string): 
      result = [] 
      n = len(string) 
      for l in range(n, 0, -1): 
          for i in range(n-l + 1): 
              result.append(string[i:i+l]) 
      return result 
  
  assert substring_in_order("abc")==['abc', 'ab', 'bc', 'a', 'b', 'c']
  assert substring_in_order("xy")==['xy', 'x', 'y']
output: def substring_in_order(string): 
      result = [] 
      n = len(string) 
      for l in range(n, 0, -1): 
          for i in range(n-l + 1): 
              result.append(string[i:i+l]) 
      return result