I have an imported type

```
type ExternalType struct {
   quantity int
}

type ExternalArray []*ExternalType

```

I want to be able to implement the sort interface for ExternalArray for so that I sort it by quantity.

However, I am not sure how I could do that?

A concrete example is this:

<https://play.golang.org/p/bEPtJ8NHQK>