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true-list type specifier

Since we seem to be on the subject of type specifiers, let me add this
question.  Given the following code

(defun foo (my-list)
  (declare (list my-list))
  (member 'my my-list))

and a compiler that has been told to turn off type-checking where
licenced by user declarations.  One might assume that MEMBER need not
do a LIST check on each CDR of MY-LIST.  But this is not the case, the
above declaration merely licences the compiler to assume that MY-LIST
is NIL or a CONS, but a type check should still be done on the CDR of
MY-LIST (when it is a CONS), and on its CDR, etc.

My question is:  How does one declare that MY-LIST is a true list?
For the purpose of discrimination, I suppose the following would
(deftype true-list ()
  '(or null (and cons (satisfies true-list-p))))

(defun true-cons-p (cons)
  (declare (cons cons))
  (null (last cons)))

But this would not be usable for purposes of declaration (unless one
was dealing with an AI compiler :->).

So I propose that the type specifier true-list be introduced.  This
would then allow compilers to validly substitute a NULL test for a
NULL/CONSP test in the case mentioned above.

	-- Nick