title: How to implement "appendFile" function?
body: <p>I can use the following function to overwrite a text file:</p>
  
  <pre><code>let writeFile ~filename:fn s =
  let oc = open_out fn in
      output_string oc s;
      close_out oc ;;
  </code></pre>
  
  <p>Howeve, i donot know how to append a line to a text file ?</p>