Boost-Mpl documented but missing template

/*
 * main.cpp
 *
 *  Created on: Jul 8, 2013
 *      Author: didier_m
 */

#include <string>
#include <iostream>
#include <boost/mpl/at.hpp>
#include <boost/mpl/map.hpp>
#include <boost/mpl/pair.hpp>

// Minimal policy based design example (inspired from the freakycpp.com article)
// I'll define a Foo class (let's imagine it actually does something)
//  some member functions and include policy-defined logging.

// This is de default policy map:
// It is just an empty map. Policy consumers may define their own defaults.
typedef boost::mpl::map<> DefaultPolicy;

// This is the key used to identify the logging policy.
// PK == policy key
class LogPK;

// The default logging policy (noop)
struct DefaultLogP
{
    static void log(const std::string& message) {}
};

// Custom logging policies
struct StdCoutLogPolicy
{
    static void log(const std::string& message) {
        std::cout << message << std::endl;
    }
};
struct StdCerrLogPolicy
{
    static void log(const std::string& message) {
        std::cerr << message << std::endl;
    }
};

template<class TPolicy=DefaultPolicy> class Foo
{
    // Let's resolve the logging policy and make a typedef to it:
    typedef typename boost::mpl::at<TPolicy, LogPK, DefaultLogP>::type LoggingPolicy;

public:
    static void doBar(const std::string& message) {
        LoggingPolicy::log(message);
        // ...
    }
};

int main()
{
    // Once with the noop policy:
    Foo<>::doBar("This message isn't supposed to show up");

    Foo<boost::mpl::map<boost::mpl::pair<LogPK, StdCoutLogPolicy> > >
        ::doBar("Log on std::cout");
    Foo<boost::mpl::map<boost::mpl::pair<LogPK, StdCerrLogPolicy> > >
        ::doBar("Log on std::cerr");
}