NotNull struct : ready to use?

[Namespace]

Next week begins my university project and I want to write it in 
D (with my own 2D framework, which is called "Dgame" [my own 
clone of SFML, and still in alpha stage]).
Because I want to test my NotNull structure (and to avoid nasty 
access violations) and since my team is completely new to D and 
does not yet have much programming experience, I hereby wish to 
post comments, improvement suggestions or criticism for my 
current NotNull structure.
What can I do better / differently?

NotNull.d:
http://dpaste.dzfl.pl/9ad9c8b6

Test_NotNull.d
http://dpaste.dzfl.pl/d70ec4e6

Thank you in advance.