  This essay is dedicated to Prof. K.K. Phua on the occasion of his 80th
birthday. While the contents of this essay are based on our recent
papers$^{1,2}$ published in the International Journal of Modern Physics A, I
have added many personal opinions, so that I am solely responsible for all
statements in this essay.
  1. S. Aoki, T. Onogi and S. Yokoyama, Int. J. Mod. Phys. A36 (2021) no.10,
2150098. 2. S. Aoki, T. Onogi and S. Yokoyama, Int. J. Mod. Phys. A36 (2021)
no.29, 2150201.
