[boost] [graph] [iterator] use of iterator_facade in leda_graph.hpp

From leda_graph.hpp: class out_edge_iterator : public iterator_facade<out_edge_iterator, leda::edge, bidirectional_traversal_tag, const leda::edge&, const leda::edge*> Can someone explain to me the parameter "const leda::edge*"? http://www.boost.org/libs/iterator/doc/iterator_facade.html#difference says the last template parameter is the type for the difference between two iterators, how can this be a pointer? Shouldn't it just be the default std::ptr_diff_t? Cheers, Jens