Proposed definition of SUBST

[STEELE at CMU-20C]

Here is a tentative definition of SUBST for inclusion in the white pages:
(defun subst (old new tree @key (test #'eql testp)
		                (test-not nil test-not-p)
				(key #'(lambda (x) x)))
  (cond (test-not-p
	 (if testp @r[<signal-an-error>]
	     (subst old new tree
		    :test #'(lambda (x y) (not (funcall test-not x y)))
		    :key key)))
	((atom tree)
	 (if (funcall test old tree) new tree))
	(t (let ((a (subst old new (car tree) :test test :key key))
		 (d (subst old new (cdr tree) :test test :key key)))
	     (if (and (eq a (car tree)) (eq d (cdr tree)))
		 tree
		 (cons a d))))))

I had two problems.  One is that :TEST-NOT is a pain to implement.
Either I have to use the subterfuge I did here, which is clumsy or slow,
or else I have to implement SUBST twice, once for the version that
uses TEST and once for the one that uses TEST-NOT.  Actually, what
I need is a function XOR, taking two boolean values (NIL/not-NIL)
and returning their xor, so that I can pass a TEST-NOT-P flag and
xor it with the result of the predicate.
The other problem is that I wish there were a standard identity function
for use in initializing the :KEY parameter.  (I refuse to use VALUES!)
Any suggestions on this or other aspects of the above definition?
--Guy
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