struggle with ports on step compare

XProc processor used calabash-0.9.11

a) having a working pipeline: showing the relevant snippet, writing  
some xml to the file system.

   <p:identity name="compare">
             <p:input port="source">
                 <p:inline>
                     <test/>
                 </p:inline>
             </p:input>
    </p:identity>
    <p:store name="write2file">
              <p:input port="source">
                  <p:pipe port="result" step="compare"/>
              </p:input>
              <p:with-option name="href" select="concat('/Users/paul/ 
Desktop/R&amp;D/xmlschema/',$xml,'.err')"/>
    </p:store>

b) pipeline throwing following error if the identity step is replaced  
by a compare step.

Error  : file:/Users/paul/Desktop/R&D/xmlschema/iterateconfig.xpl:122:  
option href unbound on p:store step named write2file and no default  
readable port.
Error  : Pipeline failed: err:XS0032: option href unbound on p:store  
step named write2file and no default readable port.  It is a static  
error if no binding is provided and the default readable port is  
undefined.
Process ended with exit code: 0

<p:compare name="compare">
             <p:input port="source">
                 <p:pipe port="result" step="tryxsd"/>
             </p:input>
             <p:input port="alternate">
                 <p:pipe port="result" step="trysch"/>
             </p:input>
             <p:with-option name="fail-if-not-equal" select="'false'"/>
  </p:compare>

<p:store name="write2file">
              <p:input port="source">
                  <p:pipe port="result" step="compare"/>
              </p:input>
              <p:with-option name="href" select="concat('/Users/paul/ 
Desktop/R&amp;D/xmlschema/',$xml,'.err')"/>
  </p:store>

The compare step is, according to me, OK, since when using it as a  
last step in the pipeline, the expected value appears on the console.
I know that the result port on compare is not primary, hence the  
explicit binding effort.

What am I missing?


Kind Regards,
Paul