** conditional tangling
   :PROPERTIES:
   :DATE:     2010-08-31
   :END:
#+begin_src emacs-lisp :results silent
  (setq tangle-tag "right")
#+end_src

*** first subheading                                                   :left:
    :PROPERTIES:
    :DATE:     2010-08-31
    :END:
#+begin_src R :tangle (and (equal (car (org-get-tags-at (point))) tangle-tag) "yes")
  "first"
#+end_src

*** second subheading                                                 :right:
    :PROPERTIES:
    :DATE:     2010-08-31
    :END:
#+begin_src R :tangle (and (equal (car (org-get-tags-at (point))) tangle-tag) "yes")
  "second"
#+end_src