** detangling example
   :PROPERTIES:
   :tangle:   yes
   :comments: yes
   :shebang:  #!/bin/sh
   :ID:       7a22cf71-6be3-4fca-a700-4c8be8237303
   :DATE:     2011-04-01
   :END:

#+name: sh-for-tangling
#+begin_src sh
  echo "this is the `sh-for-tangling' code block"
  num=`expr 1 + 1`
  echo "the value of num is $num"
#+end_src

#+begin_src sh
  echo "this is an unnamed code block"
#+end_src