在STL源码当中，如何使用一颗红黑树同时实现map和set的？

template <class Value>
struct __rb_tree_node : public __rb_tree_node_base
{
  typedef __rb_tree_node<Value>* link_type;
  Value value_field;
};
template <class Key, class Value, class KeyOfValue, class Compare,
          class Alloc = alloc>
class rb_tree {
protected:
  typedef void* void_pointer;
  typedef __rb_tree_node_base* base_ptr;
  typedef __rb_tree_node<Value> rb_tree_node;
}

template <class Key, class T, class Compare = less<Key>, class Alloc = alloc>
class map {
public:
// typedefs:
  typedef Key key_type;
  typedef T data_type;
  typedef T mapped_type;
  typedef pair<const Key, T> value_type;
  typedef Compare key_compare;
  class value_compare : public binary_function<value_type, value_type, bool> 
  {
  friend class map<Key, T, Compare, Alloc>;
  protected :
    Compare comp;
    value_compare(Compare c) : comp(c) {}
  public:
    bool operator()(const value_type& x, const value_type& y) const
    {
      return comp(x.first, y.first);
    }
  };

template <class Key, class Compare = less<Key>, class Alloc = alloc>
class set {
public:
  // typedefs:
  typedef Key key_type;
  typedef Key value_type;
  typedef Compare key_compare;
  typedef Compare value_compare;
private:
  typedef rb_tree<key_type, value_type, 
                  identity<value_type>, key_compare, Alloc> rep_type;//红黑树重命名为rep_type
  rep_type t;  // red-black tree representing set
}

template <class Key, class Value, class KeyOfValue, class Compare,
          class Alloc = alloc>
class rb_tree {
protected:
  typedef __rb_tree_node<Value> rb_tree_node;
}