for(vector<TrackIter>::const_iterator i=unlinked_endpoints.begin(); i!=unlinked_endpoints.end(); ++i)
{
start_sn = (*i)->get_snap_node(i->entry());
for(vector<TrackIter>::const_iterator i=unlinked_endpoints.begin(); i!=unlinked_endpoints.end(); ++i)
{
start_sn = (*i)->get_snap_node(i->entry());