void CellFilter::registerSubset(const CellFilter& sub) const
{
  SubsetManager::registerSubset(*this, sub);
  
  for (Set<CellFilter>::const_iterator 
         i=sub.knownSubsets().begin(); i!=sub.knownSubsets().end(); i++)
  {
    SubsetManager::registerSubset(*this, *i);
  }
}
bool CellFilter::isKnownSubsetOf(const CellFilter& other) const
{
  if (other.knownSubsets().contains(*this)) return true;
  return false;
}