commonlisp types

[Robert W. Kerns <RWK@F.ILA.Dialnet.Symbolics.COM>]

    Date: Tue, 6 Dec 88 21:46:27 PST
    From: Jon L White <jonl@lucid.com>

    re: There seems to be nothing in CLtL that answers the question:
	    "is x a legal type specifier?"

    At the meeting that founded the X3J13 committee (on 6-Dec-85), Guy Steele 
    circulated a list of "non-controversial issues" and "Clarifications"
    which included the following addition [typos faithfully reproduced]:

      "(*) 51 Add a newefunction TYPE-SPECIFIER-P that is true of valid type
       specifiers and fals of all other Lisp objects.  Note that the use of
       DEFSTRUCT and DEFTYPE can change the behavior of TYPE-SPECIFIER-P over
       time."

    Sad to say, this and many other "non-controversial" items included in
    Guy's list of "Clarifications", has never been brought up in the X3J13
    "Cleanup" subcommittee.  However, Lucid's 3.0 release includes such 
    a function.

How do you define "valid type specifier"?

(deftype foo (x)
   (car x))

(typep 'yow '(foo a))

==> Error taking CAR of the symbol A.

(type-specifier-p '(foo a))

==> ?

I would guess probably T, but then, how do you word the definition?
If not, how do you define it?