I have a question about the requirements of the MultiPassInputIterator concept as used in the Boost Graph Library. Does the ++first_iter == ++second_iter requirement hold across calls to out_edges(vertex, g)? Or put another way, does out_edges always have to return an iterator with same ordering of returned edges? The difference is between: graph_traits<graph>::out_edge_iterator i = out_edges(vertex, g); graph_traits<graph>::out_edge_iterator j = i; ++i == ++j; // clearly required, but sufficient? and graph_traits<graph>::out_edge_iterator i = out_edges(vertex, g); graph_traits<graph>::out_edge_iterator j = out_edges(vertex, g); ++i == ++j; // also required? Strangely enough, I have an application where the second case is (currently) not guaranteed (its a custom graph class where the out_edge_iterator is constructed on-the-fly). THK -- Timothy H. Keitt http://www.keittlab.org/