Sealed types

[Brian Goetz]

>
> Or, if not additive, but we end up reusing the `final` keyword in the 
> way shown at the bottom of this email, then we could at least allow 
> `permits /<bikeshed>/, TypeA, TypeB` which is maybe nearly as good.

In light of this morning's observation about hyphenated keywords ... 
there's a lot in this thread about why it seemed more attractive to 
retcon final for sealed-ness rather than create a new conditional 
keyword.  But it was a bit confusing (because final already has 
associations), and we ran into the problem that

     final class X { }

already means something, which deprived us of the opportunity to infer a 
permits clause.  Switching to something derived from `final` (such as 
`semi-final`) restores that, while keeping the associations with final:

     semi-final class A
         permits X, Y, Z { ... }

     non-final class X extends A { ... }

     class Y extends A { }  // implicitly final

     semi-final class B
         /* inferred permits clause */ { ... }

I think this is the best of both worlds; we clearly connect to finality, 
but don't directly overload it.
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