Method: (P-POSITION T PERSISTENT-ARRAY)

Source

(defmethod p-position (value (vector persistent-array)
                             &key (key #'identity) (test #'p-eql)
                             (start 0) (end nil))
  (check-p-vector vector 'p-position)
  (loop for i from start below (or end (p-length vector))
        do (let ((elt (funcall key (p-aref vector i))))
             (when (funcall test value elt)
               (return-from p-position i))))
  ;; Return nil if not found
  nil)