Equation Icon  1( ab _L ab) cd A B cd S ( cd ) f := fabcd 2 e - i e e = ig Bf Acde = R - 2idc e Ad , (22) 1 ( ) ( ) F := Fabcd-- eab- i _L eab ecd = igABF BAcdecd = f + hh'- 2s'easeb qab + ieab . (23) 2