assert(el); typename ForwardMap::const_iterator p = forwardMap.find(el); if (p != forwardMap.end())
assert(elem); typename BackwardMap::const_iterator p = backwardMap.find(elem); if (p != backwardMap.end())
  assoc(const typename Model::Element& el) const  {    assert(el);    typename ForwardMap::const_iterator p = forwardMap.find(el);    if (p != forwardMap.end())      return (*p).second;    else      return 0;  }