code: func ObjectIdHex(s string) ObjectId {
  	d, err := hex.DecodeString(s)
  	if err != nil || len(d) != 12 {
  		panic(fmt.Sprintf("Invalid input to ObjectIdHex: %q", s))
  	}
  	return ObjectId(d)
  }
docs: // ObjectIdHex returns an ObjectId from the provided hex representation.
  // Calling this function with an invalid hex representation will
  // cause a runtime panic. See the IsObjectIdHex function.