input: def substring_in_order(string: str) -> List[str]:
      """
      Generate a Python program which takes a string as input and prints all of its 
      substrings in order from longest to shortest length.
      >>> substring_in_order("abc")
      ['abc', 'ab', 'bc', 'a', 'b', 'c']
      >>> substring_in_order("xyz")
      ['xyz', 'xy', 'yz', 'x', 'y', 'z']
      """
output:     substrings = []
      length = len(string)
      for i in range(length, 0, -1):
          for j in range(length - i + 1):
              substrings.append(string[j:j+i])
      return substrings