・Corrected a misprint in Remark 3.2.4, (iv) ("either of following" ---> "either of the following") ・Added a supplementary explanatory phrase concerning "the trivial Galois extension" in Example 3.5, (i) ・Reworded the conclusion stated in the final sentences of Corollary 3.7, (ii), (iii), in terms of poly-isomorphisms ・Slightly rearranged the wording in the second to last sentence of Corollary 3.9, (i) ・Added a supplementary explanatory phrase to the discussion preceding the second display of Example 4.3, (i) ・Deleted the word "readily" from the discussion of Remark 4.3.3, (ii) ・Deleted an unnecessary "of" from the final sentence of Remark 4.5.1, (ii)