top_stree
struct stree *top_stree(struct stree_stack *stack);
stree = clone_stree(top_stree(fake_cur_stree_stack));
merge_stree(&stree, top_stree(fake_cur_stree_stack));
return clone_stree(top_stree(cond_true_stack));
return clone_stree(top_stree(cond_false_stack));