title: In emacs lisp how can I append a string to a file that I don't like to open?
body: <p>I need to <strong>append</strong> a string <code>S</code>(belonging to no buffer) to a special file <code>F</code>, but since I hope this operation takes as little time as possible I don't want <code>F</code> opened as a buffer.</p>
  
  <p>AFAIK, there is a built-in function in emacs called <code>write-region</code>, but this requires the content to be written into <code>F</code> inside one of the buffers(while in my case there is no guarantee that <code>S</code> should be such a string). How can I make it?</p>