Description of conflicts of interest: There is no conflict of interest.
Sources of financial support: None
Description of the process by which someone else could obtain the data and computing code required to replicate the results reported in the submission: All resources of data and methods of analysis are described in the letter.
Acknowledgments: The author is grateful to Professor Yutaka Hamaoka of Keio University for instructive discussions.
Corresponding author’s name: Toshiko Kato, Mailing address: Kangoku-cho, Nara, Japan, Telephone number: E-mail address: email@example.com