Tristate - wanna?

[Andrei Alexandrescu via Digitalmars-d]

On 03/26/2016 10:36 PM, Alex Parrill wrote:
> N-state logic isn't just about probabilities either. According to
> Wikipedia, Bochvar's three-valued logic has an "internal" state, where
> any operation with `internal` results in `internal` (similar to NaN).
> More broadly, the values and operations between them could be whatever
> the mathematician or developer wants, so a truth table is one of the
> ways to generally specify an operator.

BTW there's a tri-state logic type in std.allocator. Care to take a look 
at how it might be improved? -- Andrei