多态的本意：是抽象类型具体实现的多样化；
然后对具体类型的行为，作出或相同或不同的解释；
多态是一个类型理论的名词；对抽象类型的行为作出了解释；
抽象类型的指代作用举例：
这个牛是一头白牛，这个牛（这头白牛）........
The most commonly recognised major classes of polymorphism are:
- Ad hoc polymorphism: defines a common interface for an arbitrary set of individually specified types.
- Parametric polymorphism: when one or more types are not specified by name but by abstract symbols that can represent any type.
https://en.wikipedia.org/wiki/Polymorphism_(computer_science)#Static_and_dynamic_polymorphism