PRE?
--------------------------------------------
void sortR(link h, void (*visit)(Item)){
   if (h==z) return;
   visit(h->item);   
   sortR(h->l,visit);
   sortR(h-r,visit);
}

void STsort(void (*visit)(Item)){
   sortR(head,visit);
}
--------------------------------------------
POST?
void sortR(link h, void (*visit)(Item)){
   if (h==z) return;
   sortR(h->l,visit);
   sortR(h-r,visit);
   visit(h->item);   
}

void STsort(void (*visit)(Item)){
   sortR(head,visit);
}
--------------------------------------------