<p>
Output to the input port is implicitly dropped, that is,
<code>output</code> becomes a no-op if <code>outport</code> ==
- <code>inport</code>.
+ <code>inport</code>. Occasionally it may be useful to override
+ this behavior, e.g. to send an ARP reply to an ARP request; to do
+ so, use <code>inport = "";</code> to set the logical input port to
+ an empty string (which should not be used as the name of any
+ logical port).
</p>
</dd>