The |result port for p:compare?

using:  Calabash 0.6.0

When I use a pipeline with a p:compare step, Calabash gives this error:
Input |result unbound on pipeline step named main and no default binding
available.

I can make the error go away by inserting this:

<p:input port="|result"/>

But when I look at the spec
(http://www.w3.org/TR/2008/WD-xproc-20080814/#c.compare), I don¹t see this
port mentioned.

What am I missing? 

TIA

James Garriss
http://garriss.blogspot.com



PS Here¹s my pipeline:

<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" name="main">
    <p:input port="source" primary="true">
        <p:document href="Bookstore.xml"/>
    </p:input>
    <p:input port="alternate">
        <p:document href="BookStoreDupe.xml"/>
    </p:input>
    <p:input port="|result"/>
    <p:output port="result"/>
    <p:compare fail-if-not-equal="true">
        <p:input port="source">
            <p:pipe step="main" port="source"/>
        </p:input>
        <p:input port="alternate">
            <p:pipe step="main" port="alternate"/>
        </p:input>
    </p:compare>
</p:declare-step>