|  |  |  | Singletons | 
Singleton types match exactly one value using ==. Singletons are the most specific types.
| <singleton> | (<type>) | C | 
| t= | (x|<any> => <singleton>) | G | 
| returns singleton constrained to x. | ||
| type-object | (x|<singleton> => <any>) | G | 
| object that singleton type matches. | ||
|  |  |  | Singletons |