0001 template <typename T, typename V> void inorder2(BNP<T> x, V& visit) {
0002    auto first = [](auto x) { return eov(x); }; //中序的首节点即EOV；其他遍历需相应重写
0003    for (x = first(x); x; x = x->succ()) //依次
0004       visit(x->data); //枚举所有节点
0005 } //inorder2