input: Write a function to process user input commands as shown in the given code and handle 'exit', 'help', and unrecognized commands.
  assert process_user_input('exit')=='SystemExit'
  assert process_user_input('help')=='This is a helpful message.'
  assert process_user_input('hello')=='Command not recognized, try again.'
output: def process_user_input(command):
      if command == 'exit':
          return 'SystemExit'
      elif command == 'help':
          return 'This is a helpful message.'
      else:
          return 'Command not recognized, try again.'