Adding p:group gets rid of ERR : Unbound primary output.. why?

Hi,

Why does adding a p:group get rid of the following error message?

E [Calabash XProc] ERR : Unbound primary output: [output result on !1.29]

Without the p:group in the following bit of code, in a larger pipeline, I'm
getting the above error message. Something in my gut told me group the
content and then no more error message is displayed in oxygen. Why the error
and why did it go away when grouped?

 <p:group>
    <p:identity name="PutCopyCitySpecificWikiDump"/>

    <p:xslt name="identifyDifferentOldNewOfficeCodes" version="1.0"
oh:doc="run xslt to append the office code list">
      <p:input port="stylesheet">
        <p:document href="../xslt/identifyDifferentOldNewOfficeCodes.xsl"/>
      </p:input>
      <p:with-param name="hrefRDF" select="$hrefRDF"/>
      <p:with-param name="regexSelectSchemaTypes"
select="'\{\{Schema\s+(E(lement|numeration)|Type|Attribute)'"/>
      <p:input port="parameters">
        <p:empty/>
      </p:input>
    </p:xslt>


    <oh:save-output step-name="saveidentifyDifferentOldNewOfficeCodes">
      <p:with-option name="save" select="'true'"/>
      <p:with-option name="href"
select="$hrefIdentifyDifferentOldNewOfficeCodes"/>
    </oh:save-output>

    <p:identity name="GetCopyCitySpecificWikiDump">
      <p:input port="source">
        <p:pipe port="result" step="PutCopyCitySpecificWikiDump"/>
      </p:input>
    </p:identity>
  </p:group>

Regards

-- 
Alex
-----
Currently:
Freelance Software Engineer 6+ yrs exp
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