Accessing a Collection in p:variable

I'm using Morgana 1.4.5 on Windows.
I want load a collection of XML files into a variable and then retrieve information from one specific file but have been unable to get it to work. I've a simplified example of the code I've got so far, but here's a summary of the steps:
1. Establish source and result ports. Source file is input-main.xml2. Create variable "new_s_value" to get collection of files in subfolder "files"; use the unique root element of the target file to identify the correct file in the collection and then get the desired attribute value from the root element3. Re-write the value of the target attribute "s" in the source file using the new value from the collection (XSLT transform)4. Output the new version of the input
As you can see below (also in files in zip attachment), all this works if I don't use a collection on the variable "new_s_value". But that's only a test. I need to use a collection on variable "new_s_value" because I won't ever be able to know the name of the target file. But I'm missing something about how to access a collection on the variable. The error I get from Morgana is
"Error in XPath expression '/c/@s': Context item is needed, but not provided: 1 items found"
I've been unable to determine how to provide the missing XPath context; I thought the @href would provide it but apparently not.
XPROC:<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" version="3.0" name="main-step">        <p:input port="source" primary="true" href="input-main.xml" />        <p:output port="result" primary="true"  sequence="true"/>        <!-- this works but is only a test -->    <!--<p:variable name="new_s_value" href="files/z3.xml" select="/c/@s" />-->        <p:variable name="new_s_value" collection="true" href="{collection(files)}" select="/c/@s" />            <p:xslt message="new s value: {$new_s_value}" parameters="map{'new_s_value': $new_s_value}">       <p:with-input port="stylesheet" href="rewrite-s-value.xsl"/>   </p:xslt>       <p:store href="stored/rewrite-s-value.xml"></p:store>    </p:declare-step>
INPUT FILE input-main.xml<sample-source s="z5">    <div>        <section s="z5">            <title>Section level 1 title content.</title>            <div>                <section s="z5">                    <title>Section level 2 title content.</title>                </section>            </div>        </section>    </div></sample-source>
COLLECTION FILES IN SUBFOLDER "files"z1.zml<z s="no1">    <p>Z document with a paragraph.</p></z>
z2.xml<y s="no2">    <p>Y document with a paragraph.</p></y>
z3.xml<c s="t9">    <p>C document with a paragraph.</p></c>
XSLT rewrite-s-value.xsl<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"    xmlns:xs="http://www.w3.org/2001/XMLSchema"    xmlns:math="http://www.w3.org/2005/xpath-functions/math"    exclude-result-prefixes="xs math"    version="3.0">            <xsl:param name="new_s_value" as="xs:string">default</xsl:param>        <xsl:template match="*">        <xsl:copy>            <xsl:if test="@s">                <xsl:attribute name="s" select="$new_s_value"/>            </xsl:if>            <xsl:apply-templates/>        </xsl:copy>    </xsl:template>        </xsl:stylesheet>
Thanks,
Don