I think the previous poster's use of the phrase "reach into your package" was a bit colorful. You're never actually modifying the internal code of a library. Yo

I think the previous poster's use of the phrase "reach into your package" was a bit colorful. You're never actually modifying the internal code of a library. You're only extending a generic function from a library to work on your own custom type. So that extension only affects code that uses your new custom type. It's akin to using class inheritance in OOP languages.