Can a D library have some types determined by the client program?

[Liam McGillivray]

Update on two things:

One is that I now better understand what it means that D objects 
are "reference by default". This means that references *can* be 
null if they are declared with a class. In my commits last night, 
I have changed many pointers into references. I think my time 
will be smoother from now on, spending far less time trying to 
debug segfaults.

Secondly, I found out that interfaces can't have variables. 
What!? That's crazy! Why wouldn't they? They totally should. 
Doesn't this mean that I will need to use getter and setter 
functions instead of direct access when using interfaces? I don't 
like this.