Open-Closed Principle

[長話短說]

Open-Closed Principle : open for extension / close for modification (封閉/開放原則)
把程式寫得有彈性，好擴展功能
把程式包裝得夠隱蔽，以避免無心或惡意的竄改

[細細說來]
...(working on it)...