Compound steps with unconnected output ports

Hi,

Suppose I have this p:pipeline:

<p:pipeline>
  <p:output port="foo" primary="false"/>
  <p:identity/>
</p:pipeline>

The "foo" output port is not connected to anything. Also, because it is
not primary, it will not get connected to the primary output port of
p:identity.

Is this allowed? I can't find anything in the spec that makes this an
error, or that says what happens when I actually refer to "foo" when I
call the pipeline.

Also, would there be any difference if I declared "foo" as
sequence="true"?

The only thing I found is in section 2.2:

"Within a compound step, the declared outputs of the step *can* be
connected to: ..."
(Notice that the sentence uses "can", not "must".)

Regards,
Vojtech