private:
  typedef vector<bbox> iter_memo_t;

public:
  typedef typename iter_memo_t::const_iterator const_iterator;

private:
  mutable iter_memo_t iter_memo;

public:
  int setsize() const {
    iter_memo_t im;
    serialise(im);
    return im.size();
  }
  const_iterator begin() const {
    iter_memo.clear();
    serialise(iter_memo);
    return iter_memo.begin();
  }
  const_iterator end()