Swift允许定义我们自己的优先级组以用于我们的自定义运算符。有一件事我很难理解，那就是bool型assignment属性。来自Swift编程语言(Swift3.1):Theassignmentofaprecedencegroupspecifiestheprecedenceofanoperatorwhenusedinanoperationthatincludesoptionalchaining.Whensettotrue,anoperatorinthecorrespondingprecedencegroupusesthesamegroupingrulesduringoptionalch