Laura's solution is here (as a .GIF image):
http://camoo.freeshell.org/20.15.gifHowever, it seems to me (on a casual reading) that it contains several major errors.
In particular,

is supposed to be antisymmetric, so the statement:
Quote:

is nonzero only if
c is the

index
is incorrect. Furthermore the equation:
manifestly makes no sense, as there are two free indices on the left, and none (or one, if
d is unsummed) on the right. Finally:
Quote:

and

is also wrong, since the two right hand sides are equal, but

.
I have posted my solution as Exercise [20.15]b; hopefully that is error-free!