code: func (m IntMapper) ID(ii ...interface{}) (rowIDs []int64, err error) {
  	i := ii[0].(int64)
  	externalID := m.Res
  	if i < m.Min || i > m.Max {
  		if m.allowExternal {
  			return []int64{externalID}, nil
  		}
  		return []int64{0}, fmt.Errorf("int %v out of range", i)
  	}
  	return []int64{i - m.Min}, nil
  }
docs: // ID maps an int range to a rowID range