//  (C) Copyright Jeremy Siek 1999.

 // Distributed under the Boost Software License, Version 1.0. (See

 // accompanying file LICENSE_1_0.txt or copy at

 // http://www.boost.org/LICENSE_1_0.txt)

 #ifndef BOOST_TREE_STRUCTURE_HPP

 #define BOOST_TREE_STRUCTURE_HPP

 namespace boost {

   template <class T>

   struct tree_traits {

     typedef typename T::node_descriptor node_descriptor;    

     typedef typename T::children_iterator children_iterator;    

   };

   template <class Tree, class TreeVisitor>

   void traverse_tree(typename tree_traits<Tree>::node_descriptor v,

                      Tree& t, TreeVisitor visitor)

   {

     visitor.preorder(v, t);

     typename tree_traits<Tree>::children_iterator i, end;

     tie(i, end) = children(v, t);

     if (i != end) {

       traverse_tree(*i++, t, visitor);

       visitor.inorder(v, t);

       while (i != end)

         traverse_tree(*i++, t, visitor);

     } else

       visitor.inorder(v, t);

     visitor.postorder(v, t);

   }

   struct null_tree_visitor {

     template <typename Node, typename Tree> void preorder(Node, Tree&) { }

     template <typename Node, typename Tree> void inorder(Node, Tree&) { }

     template <typename Node, typename Tree> void postorder(Node, Tree&) { }

   };

 } /* namespace boost */

 #endif /* BOOST_TREE_STRUCTURE_HPP */