it's a courtesy, not a must.

所有跟帖： 

• I am talking about being polite, not must nor must not -MoonRiverMe- ♂ (80 bytes) () 01/28/2019 postreply 12:35:31

• and you don't have to be a polite person to hang around... -borisg- ♂ (0 bytes) () 01/28/2019 postreply 12:42:41

• Now Duke says must not. You may accept it, not us. -伊敏- ♀ (0 bytes) () 01/28/2019 postreply 12:56:24

• what made you think I accepted it? -MoonRiverMe- ♂ (65 bytes) () 01/28/2019 postreply 13:19:16