[Boost-users] [graph] edge_descriptor invalidation

Hello everyone! I'm asking myself, why edge-descriptors become invalid if I remove the edge and reinsert it? In my case the graph is represented by an adjacency list with vecS storage for vertices and edges. If I'm not mistaken, then the edge_descriptor's are just represented by pairs of vertex indices, right? So why are edge_descriptors invalid if I delete these edges, reinsert the edges and then want to use the old edge_descriptors to address those recreated edges? Most interestingly, a very cumbersome edge(source(old_descriptor, graph), target(old_descriptor, graph), graph).first gives me perfectly valid edge_descriptors again after removal and insertion. I'm confused. Thanks for any help in advance. Sebastian Weber