Keyword to avoid not null references

[Namespace]

On Saturday, 21 April 2012 at 23:18:40 UTC, Namespace wrote:
> I see you're right. Great! But it is only on git hub and not in 
> the standard typecons header in 2.059, right?

But one thing: i missed there an invariant which check if t is 
null.
If i write "f.t = null;" no error occured.