The metatype conflict can be avoided by manually creating the needed metaclass for C
为 C 手工创建必需的元类，可以避免元型冲突
2
A metatype type refers to the type of any type, including class types, structure types, enumeration types, and protocol types.
一个元类型是对任一类型(包括类类型，结构类型，枚举类型以及协议类型)的类型的引用。
3
Users should understand the cause and the resolution of metatype conflicts. This becomes essential when you want to use more than one custom metaclass.