Saxon error with XHTML5 stylesheets when invoked through Calabash XProc

[graeme@heliocentrik.net]

Hi all,
I'm trying to build XHTML5 output through the DocBook 1.78.1 XSL
stylesheets. My transformation is being invoked via Calabash XProc.

I can successfully invoke 'docbook/xhtml5/docbook.xsl' to create
(non-chunked) XHTML5 output with tools like xsltproc, but Calabash gives
the following error:

System ID: .../docbook/xhtml/chunker.xsl
Scenario: doc-generate
XProc file: doc-generate.xpl
Engine name: Calabash XProc
Severity: error
Description:  Processing terminated by xsl:message at line 126 in
chunker.xsl
Start location: 126:0

The underlying Saxon error is:

E [Saxon6.5.5] Variable chunk.quietly has not been declared

I'm guessing this is another Saxon versioning/extension function
availability issue, since lines 126 to 127 read as follows:

  <xsl:choose>
    <xsl:when test="element-available('exsl:document')">

My XProc XSLT step includes the option to specify XSLT 1.0, but removing
it seemingly makes no difference:

<p:xslt name="make-unprofiled-html">
                <p:input port="stylesheet">
                    <p:document href../../docbook/xhtml5/docbook.xsl"/>
                </p:input>
                <p:input port="parameters">
                    <p:empty/>
                </p:input>
                <p:input port="source">
                    <p:pipe port="current"
                    step="output-consolidated-docbook-unprofiled"></p:pipe>
                </p:input>
                <p:with-option name="version" select="1.0" />
            </p:xslt>


I'd be grateful for any guidance on how to resolve or work around this
issue.

Regards,

Graeme