ERRATA 
Page 137, 2nd line from bottom : For A, B, C, D read D, C, B, A. 
Page 138, 2nd line from top : For A read D. 
„ 3rd line from top : For B, 0, D read C, B, A. 
„ 4th line from top: For B'. O', D' read C', B', A'. 
