・Added definitions of E^\times, R^2\times to the fifth paragraph of \S 1 ・Slightly modified the paragraph in \S 1 concerning the natural embedding into a one-dimensional  complex projective space ・Slightly modified the second paragraph of \S 4 ・Added a supplementary explanation to the third to last paragraph of \S 5 ・Slightly modified the phrasing of the first sentence of the second to last paragraph of \S 5 ・Slightly modified Fig. 8