"""The type of the `null` value. Any union type of form 
   `Null|T` is considered an _optional_ type, whose values
   include `null`. Any type of this form may be written as
   `T?` for convenience.
   
   The `if (exists ... )` construct, or, alternatively,
   the `assert (exists ...)` construct, may be used to
   narrow an optional type to a _definite_ type, that is,
   a subtype of `Object`:
   
       String? firstArg = process.arguments.first;
       if (exists firstArg) {
           print("hello " + firstArg);
       }
   
   The `else` operator evaluates its second operand if 
   and only if its first operand is `null`:
   
       String name = process.arguments.first else "world";
   
   The `then` operator evaluates its second operand when
   its first operand evaluates to `true`, and to `null` 
   otherwise:
   
       Float? diff = x>=y then x-y;"""
see (`value null`)
by ("Gavin") 
shared abstract class Null() 
        of null
        extends Anything() {}

"The null value."
by ("Gavin")
shared object null extends Null() {}