remove(const typename Model::Element& el)
remove(class Element* elem)
  remove(const typename Model::Element& el)  {    assert(el);    typename ForwardMap::iterator p = forwardMap.find(el);    if (p != forwardMap.end())      {	backwardMap.erase((*p).second);	forwardMap.erase(p);	return true;      }    else      return false;  }