ENDP

[Kent M. Pitman <KMP at MIT-MC>]

Couldn't you just have written this?
 
(defun endp (thing &optional list)
  (cond ((consp thing) nil)
	((null thing) t)
	(t (cerror ':improperly-terminated-list
		   "The non-null atom ~S terminated a list~@[, ~S]."
		   thing list))))

In any case, I definitely do not like to see functions haired up with
all kinds of funny args that ideosyncratic things. There are zillions
of functions which have a potential for erring and if they all take args
of fun things to make the error message more readable, the language 
definition will be considerably more cluttered. I would want to understand
some theory of when it was appropriate to add such args to things and when
it wasn't before I thought it was a good idea to put this one in.