avl_iter_begin
void avl_iter_begin(AvlIter *iter, struct stree *avl, AvlDirection dir);
for (avl_iter_begin(&(iter), avl, direction); \
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);