Earlier versions of this paper have been presented at the seminars of Hokkaido University, Chukyo University, and Kobe University. We have greatly benefited from helpful comments and suggestions from Professors Midori Hirokawa, Kimiyoshi Kamada, Takashi Sato, Hideki Konishi, Jun Iritani, Eiichi Miyagawa, and other seminar participants. We also thank the editor, Alessandro Cigno, and two anonymous referees of this journal for their helpful comments. All remaining errors are our own responsibility.