This clears any previous output to the stream. If the stream is omitted or 0 the stream is the HTTP client stream of the calling procedure.
All output from VSP page procedures is buffered into a local string stream before being sent out. This is done so as to support the HTTP/1.1 required content length and to allow recovery from errors.
A VSP page excerpt.
<p>Nope. You won't see this.</p>
<?vsp
http ('<p>This line will contribute to your local entropy.</p>');
http_rewrite ();
http ('<p>Now THIS is what you wanted to see, isn't it?</p>');
?>