new_contents: def shell_sort(a_list):
      """Shell Sort algortihm."""
      pass
  
  
  def main():
      a_list = [54, 26, 93, 17, 77, 31, 44, 55, 20]
      print('a_list: \n{}'.format(a_list))
      print('By Shell Sort: ')
      shell_sort(a_list)
      print(a_list)
  
  
  if __name__ == '__main__':
      main()
  
subject: Add alg_shell_sort.py