Nominal Typing
[Duck Typing](https://texonom.com/duck-typing-2eb87dea7f0e436180f4a416957accf8) [Invariance](https://texonom.com/invariance-d3ff735997ad4ec082c35fb191b5af47) 


Structural Typing 보다 훨씬 엄격한 Nominal Typing
구조가 같아도 다른 클래스면 할당안댐










