#xyzzy cons の CAR-TYPE/CDR-TYPE を再帰的に指定する typespec で plist

gistfile1.lisp

(deftype plist (&optional key-type value-type)
  (let* ((tail (list value-type nil))
         (spec `(or null
                    (cons ,key-type
                          (cons ,@tail)))))
    (setf (cadr tail) spec)
    spec))

(si:canonicalize-type '(plist keyword number))
=> #1=(or null (cons keyword (cons number #1#)))