What does Z["ack"] = 5 mean?

[Tim W Smith]

Just because something is only mentioned in one place in K&R is no
excuse for a compiler writer to miss it!  Before someone writes a
compiler that other people are going to use, that person should at
least have read *ALL* of the book. 

Can anyone give a reasonable excuse for a compiler writer not to
be aware of A[B] and B[A] meaning the same thing?

						Tim Smith