Believe this article by Prof. Engelsma may be useful in this discussion:
http://www.prca.org/prtj/nov2006.pdf
In it you can read that nowhere do we deny a covenant relationship between God and Adam.
What we do deny is the view that Scripture teaches that Adam could have merited heavenly, eternal life by his obedience.
Nowhere do we deny the federal and organic responsibility of us all in Adam, for eating of the tree. As such, our original sin is also to be repented of.
Richard, I did not say that the teaching of Witsius was the same as Hoeksema. But Hoeksema is standing on the shoulders of Witsius and Brakel.