ss.
 Placeholder entries are used to keep track of the preferred
 order of headers.  Headers are never actually removed from
 the list, they're converted into placeholder entries.
 New headers are added after existing headers of the same name
 (or before in the case of <code>Received</code> and
 <code>Return-Path</code> headers).  If no existing header
 or placeholder for the header is found, new headers are
 added after the special placeholder with the name ":".</div>
<dl class="notes">
<dt>Since:</dt>
<dd