FORWARD
t->bt_order = FORWARD;
if (t->bt_order == FORWARD) {
enum { NOT, BACK, FORWARD } bt_order;
#define avl_foreach(iter, avl) avl_traverse(iter, avl, FORWARD)
avl_iter_begin(&one_iter, *one, FORWARD);
avl_iter_begin(&two_iter, *two, FORWARD);
avl_iter_begin(&one_iter, implied_one, FORWARD);
avl_iter_begin(&two_iter, implied_two, FORWARD);
avl_iter_begin(&one_iter, one, FORWARD);
avl_iter_begin(&two_iter, two, FORWARD);
avl_iter_begin(&one_iter, *stree, FORWARD);
avl_iter_begin(&two_iter, filter, FORWARD);