input: Write a function to take a string as input and return a list of all its substrings in order from longest to shortest length. If the input string is empty, return an empty list.
  
  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("xyz") == ['xyz', 'xy', 'yz', 'x', 'y', 'z']
  assert substring_in_order("") == []
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