Tristate - wanna?

[Timon Gehr]

On 10/26/2013 05:42 PM, Andrei Alexandrescu wrote:
> While messing with std.allocator I explored the type below. I ended up
> not using it, but was surprised that implementing it was quite
> nontrivial. Should we add it to stdlib?
>
> Theory: http://en.wikipedia.org/wiki/Three-state_logic
>

"The term tri-state[1] should not be confused with ternary logic 
(3-value logic)."

> ...
>
>      Tristate opBinary(string s)(Tristate rhs) if (s == "&")
>      {
>          // & yields 0, 1, 4
>          return make(value & rhs.value);
>      }
>

This does not seem right. yes&unknown should be unknown.