Code review: own list implementation

I have written an own implementation of Scalas List. You find the code
on github [1].

I'm satisfied with the code of the LinkedList, but not with the
ListBuilder. The method signatures are ugly. I want to have

def add(x: A): ListBuilder[A]

instead of

def add(x: A)(f: A => B): ListBuilder[A, B]

But I don't know how to change that. I'll be happy if someone can review
it and write his opinion and maybe some improvements.

Thanks
Antoras

[1] https://gist.github.com/1126402