Nullity (was: User model stacking: current status)

[Brian Goetz]

>> * Exclamation fatigue would be very real, so assume there is some way to make `!` the default for some scope
> +1
>
> Yes, I think it's a dead end to expect users to sprinkle '!' everywhere they don't want nulls—this is usually the informal default in common programming practice, so we need some way to enable flipping the default.

On the other hand, this is on a collision course with Kevin's 
"ref-default" recommendation, which had many strong supporting reasons, 
whether this is spelled `!` or `.val`.  The "but it will be tiring for 
people to type" doesn't feel like a very good reason to flip the default 
from something that has such strong objective justifications.

(Dan was never sold on ref-default, but Kevin was, so I'll leave it to 
him to reconcile "ref-default is the right default" with "but, 
exclamation fatigue.")