Lifting the restriction on the number of public classes per file

[Brian Goetz]

> Will imports always be necessary when pattern matching on sealed types,
> or could there be something like the provision for enum switches?

We could expose a model where a sealed nest is more like an enum, and 
the nest members are more like enum constants, in which case using the 
same rules as for enums would make sense.  If we thought that was the 
right model for sealing.  If we end up not relaxing the rules regarding 
aux classes, we should consider this for switches (but it doesn't really 
address the more general API design question.)

It is true that in many other languages, sealed types are mostly 
intended for "sums of products", which is a reasonable 80/20 target.  
Though I'd rather see a deeper / more orthogonal treatment (though that 
does force us to work through more details.)

> The hardest downside to the proposal

There's not a proposal, yet...  There's just a discussion, proximately 
motivated by an external comment, but one that has been on the drawing 
board for a while, since we started to look at more concise ways to 
express sum and product types.