Un-displacing an array

Here’s a function to get the underlying array on which a displaced array is based:

(defun undisplace-array (array)
  "Return the fundamental array and the start and end positions into
it of a displaced array."
  (let ((length (length array))
        (start 0))
      (multiple-value-bind (to offset) (array-displacement array)
        (if to
            (setq array to
                  start (+ start offset))
          (return (values array start (+ start length))))))))

From an article by Erik Naggum.