title: how to check linux X11 version?
body: <p>I need to read X11 source code, since there are a number of X11 versions, like:
  X11R1
  X11R2
  ...
  X11R7.7</p>
  
  <p>how can I find out which version of X11 my ubuntu 10.04 has installed?</p>
  
  <p>thank you.</p>