What should LAST of an atom do?

[Glenn S. Burke <GSB @ MIT-ML>]

At one point, in NIL it did not accept anything but a cons.
I found that to be giving me a hard time (i don't remember where,
it may have been some NIL-only loop expansions in part), so i changed
it to accept an argument of NIL and return that.  Otherwise, it 
detects non-atomic tails (does not complain), and circularity (and
complains).  This seems reasonable, and is compatible with Maclisp.