・Corrected a misprint at the beginning of the proof of Corollary 1.2
　("together with the natural inclusion ...")

・Corrected two misprints towards the end of the proof of Corollary 1.2
　("applying the observation ...")

・Corrected a misprint at the beginning of Remark 4.5.1, (i) ("no natural ...")

・Corrected a misprint at the beginning of Definition 6.1, (i) ("any set T equipped with ...")

・Slightly modified the wording of the references to Remark 5.3.1