namespace _maps_bits {

     template <typename Value>

     struct Identity {

       typedef Value argument_type;

       typedef Value result_type;

       Value operator()(const Value& val) const {

 	return val;

       }

     };

     template <typename _Iterator, typename Enable = void>

     struct IteratorTraits {

       typedef typename std::iterator_traits<_Iterator>::value_type Value;

     };

     template <typename _Iterator>

     struct IteratorTraits<_Iterator,

       typename exists<typename _Iterator::container_type>::type>

     {

       typedef typename _Iterator::container_type::value_type Value;

     };

   }

   /// \brief Writable bool map for logging each \c true assigned element

   ///

   /// A \ref concepts::ReadWriteMap "read-write" bool map for logging

   /// each \c true assigned element, i.e it copies subsequently each

   /// keys set to \c true to the given iterator.

   ///

   /// \tparam It the type of the Iterator.

   /// \tparam Ke the type of the map's Key. The default value should

   /// work in most cases.

   ///

   /// \note The container of the iterator must contain enough space

   /// for the elements. (Or it should be an inserter iterator).

   ///

   /// \todo Revise the name of this class and give an example code.

   template <typename It,

 	    typename Ke=typename _maps_bits::IteratorTraits<It>::Value>

   class StoreBoolMap {

   public:

     typedef It Iterator;

     typedef Ke Key;

     typedef bool Value;

     /// Constructor

     StoreBoolMap(Iterator it)

       : _begin(it), _end(it) {}

     /// Gives back the given iterator set for the first key

     Iterator begin() const {

       return _begin;

     }

     /// Gives back the the 'after the last' iterator

     Iterator end() const {

       return _end;

     }

     /// The set function of the map

     void set(const Key& key, Value value) const {

       if (value) {

 	*_end++ = key;

       }

     }

   private:

     Iterator _begin;

     mutable Iterator _end;

   };