code: func (m SparseIntMapper) ID(ii ...interface{}) (rowIDs []int64, err error) {
  	i := ii[0].(int64)
  	if _, ok := m.Map[i]; !ok {
  		m.Map[i] = int64(len(m.Map))
  	}
  	return []int64{m.Map[i]}, nil
  }
docs: // ID maps arbitrary ints to a rowID range