how does invisibility impact org?

[John Kitchin <jkitchin@andrew.cmu.edu>]

Hi, I am trying to use this code to make parts of an org-file that are
not a verse block invisible. That part works fine, and I can also unhide
it. However, after that, I get errors like

org-back-to-heading: Before first headline at position 232 in buffer
selective-text.org

If I say, put the cursor in a code block and try to run it or special
edit it.

Any ideas what is going wrong here?

Thanks,

(defun flatten (LIST)
  "flattens LIST"
  (cond
   ((atom LIST) (list LIST))
   ((null (cdr LIST)) (flatten (car LIST)))
   (t (append (flatten (car LIST)) (flatten (cdr LIST))))))

(defun view-verse ()
  "Hide everything but the verse blocks."
  (interactive)
  (org-cycle '(64))
  (let ((p (append
            (list (point-min))
            (flatten (org-element-map (org-element-parse-buffer) 'verse-block
                        (lambda (vb) (list (org-element-property :begin vb)
                                           (org-element-property :end vb)))))
            (list (point-max)))))

    (add-to-invisibility-spec 'not-verse-block)
    (loop for start in p by 'cddr
          for end in (cdr p) by 'cddr
          do (overlay-put (make-overlay start end) 'invisible 'not-verse-block))))

(defun unhide ()
  (interactive)
  (remove-from-invisibility-spec 'not-verse-block)
  (ov-clear 'invisibility 'not-verse-block))


--
Professor John Kitchin
Doherty Hall A207F
Department of Chemical Engineering
Carnegie Mellon University
Pittsburgh, PA 15213
412-268-7803
@johnkitchin
http://kitchingroup.cheme.cmu.edu