Method: (P-POSITION T PERSISTENT-CONS)

Source

(defmethod p-position (value (list persistent-cons)
                             &key (key #'identity) (test #'p-eql)
                             (start 0) (end nil))
  ;; Move list to start position.
  (loop repeat start
        do (setq list (p-cdr list)))
  ;; The real work.
  (loop for i from start do
        (if (or (p-endp list) (and end (= i end)))
            (return-from p-position nil)
          (let ((elt (funcall key (p-car list))))
            (if (funcall test value elt)
                (return-from p-position i)
              (setq list (p-cdr list))))))
  ;; Return nil if not found.
  nil)