Sealed types

[Brian Goetz]

> Maybe "permits" -> "exclusivelyIncludes" or just "exclusively"? These
> seem less likely to be misinterpreted and also less likely to lead to
> any confusion with not-rare use of "permits" as a variable (in
> semaphores etc).

Good thought.

> Is there any reason except convenience to use the implicit version for
> classes in the same unit? You'd guess that without it, IDEs would soon
> offer to help fill in the lists.
>
It’s convenience for sure, but users might well think it’s a little 
excessive to have to say:

|final interface Node permits IntNode, PlusNode, MulNode, NegNode, 
ParenNode { } record IntNode(int value) implements Node; record 
PlusNode(Node left, Node right) implements Node; record PlusNode(Node 
left, Node right) implements Node; record NegNode(Node node) implements 
Node; record ParenNode(Node node) implements Node; |

When the whole sum-of-products is this compact, saying “IntNode” twice 
will feel like the same sort of Java verbosity everyone complains about. 
And in typical Visitor situations, there are more than a handful of 
subtypes, meaning that the “permits” clause will be pretty long.

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