* Conditional Tangle Example

To setup, run this block to ensure foo exists and bar doesn't.
#+begin_src sh :tangle no :results silent
  date > foo
  rm -f bar
#+end_src

Then tangle the buffer with =C-c C-v t=.

#+begin_src emacs-lisp :tangle (if (file-exists-p "foo") "yes" "no")
  (message "only tangled if %s exists %s" "foo" (file-exists-p "foo"))
#+end_src

#+begin_src emacs-lisp :tangle (if (file-exists-p "bar") "yes" "no")
  (message "only tangled if %s exists %s" "bar" (file-exists-p "bar"))
#+end_src