title: Error whille installing flex in Ubuntu 10.4
body: <p>When I type <code>sudo apt-get install flex</code> in the terminal in Ubuntu 10.4 LTS, the following message is displayed and the process terminates:</p>
  
  <pre><code>reading package lists...done
  
  building dependency tree 
  
  reading state information..done
  
  package flex is not available ....but is referred to by another package
  
  this may mean that package flex is missing, has been obsoleted or is only available from another source
  E: package flex has no installation candidate
  </code></pre>