In the displayed formula defining hi on page 63 replace "u" by "c" everywhere to the right o f the bracket. In the displayed formula defining h2 on page 63 replace "if z = d or z = e or x = 0" by "if z = d or x = 0", and also replace "if z = u and x = (a, b)" by "otherwise". Replace the displayed formula defining ha on page 63 by h3(g~, g, ux, y) = "(C(b, al)) -1 gi-l gz), al, bl) i f x = (ai, b), y = b, and C(b, al) ~ 0 (g2, al, ba) if x = (al, bl) and y = I (g?ig~, al, bl) if x ---(al, bl), y

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... = b and C(b, al) = 0 (g~ag2, al, bl) i f x = (al, bl) and y = 0 0 otherwise, i.e., x = c.