void Scene_Single::remove_feature_relation(
    Feature *f, 
    vector<Feature_Relation *> related_features)
{
  cerr << "removing feature relation" << endl;
  cerr << "size:" << related_features.size() << endl;
  vector<Feature_Relation *>::iterator it;

  for (it = related_features.begin(); it != related_features.end();it++)
  {
    if((*it)->related_feature==f)
    {
       related_features.erase(it);
       break;
     }
   }
  cerr << "size after:" << related_features.size() << endl;
}