Difference Lists for visiting trees (Prolog)

This is are some (simple) exercises in Prolog using difference lists.They should be correct, but I have to say I'm a beginner Prolog programmer... so...

inorder_dl(tree(X, L, R), Xs-Ys) :- 
        inorder_dl(L, Xs-[X|Zs]), 
        inorder_dl(R, Zs-Ys). 
inorder_dl(void, Xs-Xs). 
preorder_dl(tree(X, L, R), [X|Xs]-Ys) :- 
        preorder_dl(L, Xs-Zs), 
        preorder_dl(R, Zs-Ys). 
preorder_dl(void, Xs-Xs). 
postorder_dl(tree(X, L, R), Xs-Zs) :- 
        postorder_dl(R, Ys-[X|Zs]), 
        postorder_dl(L, Xs-Ys). 
postorder_dl(void, Xs-Xs).