non-expanded external type

[Paolo Redaelli]

I was playing with external type example wondering what's the indetended
usage of non-expanded external type.
Current snapshot tells me that: "Classes with an external type must be
expanded or deferred."

I naively put together a little example to test non-expanded external
classes that I have attached here.

Is there any documentation about those types? The wiki page on external
types is not helpful to me.

Thanks in advance
    Paolo