RE: p:choose is poorly and/or incorrectly specified

> I wonder if this is a common enough problem that we ought to add a
> primary output port to p:error, documented as never producing any
> output.

Personally, I think this p:choose/p:when/p:error scenario may be rather
common, especially if you want to write a pipeline that checks its input
before it does something interesting/dangerous.

I also did something similar to this a couple of times:

<p:try>
  <p:group>
    <p:output port="result"/>

    <!-- do something that should produce exactly one doc -->
    ...
  </p:group>
  <p:catch>
    <p:error code="...">...</p:error>
    <p:identity>
      <p:input port="source">
        <p:empty/>
      </p:input>
    </p:identity>
  </p:catch>
</p:try>


Regards,
Vojtech