Null references redux

[BCS]

Hello Walter,

> The problem with non-nullable references is what do they default to?
> Some "nan" object? When you use a "nan" object, what should it do?
> Throw an exception?
> 

They don't have a default. There semantics would be such that the compiler 
rejects as illegal any code that would require it to supply a default.

As to the user stuffing "c_empty" in just to get the compiler to shut up; 
firstly, that says the variable should not yet be declared as you don't yet 
known what value to give it and secondly either c_empy is a rational value 
or the user is subverting the type system and is on there own.