Circular lists/shared structures in org-element parse-tree

[Thorsten Jolitz <tjolitz@gmail.com>]

Hi List, 

I wonder how I can find out in a (elisp) program the points in the parse
tree (returned by org-element-parse-buffer) where shared structures are
used. 

In the read-syntax, its easy to see (especially with `print-circle' set
to non-nil):

#+begin_src emacs-lisp
  #2=(org-data nil #1=(headline (:raw-value "header 1"
   [...] :parent #2#) [...]  
#+end_src

but when processing the parse tree as a list in elisp, how can I detect the
fact that 

,------------
| :parent #2#
`------------

refers to 

,-----------------
| #2=(org-data nil
`-----------------

i.e. points back to an already existing structure?

-- 
cheers,
Thorsten