TriState

A three-valued type equivalent to Option<bool>:

rust enum TriState { Yes, No, Unknown }

A nice way to use this type is with a domain-specific type alias. For example, a spam classifier:

```rust type Spam = TriState;

trait Classify { fn classify() -> Spam; }

impl Classify for Message { /* ... */ }

// ...

match message.classify() { Spam::Yes => /* ... /, Spam::No => / ... /, Spam::Unknown => / ... */ } ```