[aw1621107]

> Where is `zero()` defined?

It's defined by whatever implements the interface.

> Why does a `Numeric` have a function to `add` two numbers?

Because Java doesn't have user-defined operator overloading, so if you want to add stuff in a generic fashion you can't rely on `+`.

> Shouldn't you add a single number to a numeric? What is the relationship between `<T>` and `Numeric<T>`? I thought `T` was a `Numeric`?

`Numeric` doesn't contain data, it just defines operations on numbers. So `Numeric<Integer>` would define operations on `int`s, `Numeric<BigInteger>` would define operations on `BigInteger`s, etc.