code: func (id ObjectId) byteSlice(start, end int) []byte {
  	if len(id) != 12 {
  		panic(fmt.Sprintf("Invalid ObjectId: %q", string(id)))
  	}
  	return []byte(string(id)[start:end])
  }
docs: // byteSlice returns byte slice of id from start to end.
  // Calling this function with an invalid id will cause a runtime panic.