pxf:copy (Calabash)

I can't get pxf:copy working properly if I use "href" as the variable name
for the target option. If I do, the file referenced in the href *option* is
overwritten with an empty file.

Here's the pipeline (example fileset (debug.xpl+image.png) also attached as
a ZIP):

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<?xml version="1.0" encoding="UTF-8"?>
<p:declare-step version="1.0" xmlns:p="http://www.w3.org/ns/xproc"
xmlns:pxf="http://exproc.org/proposed/steps/file">

    <p:import href="http://xmlcalabash.com/extension/steps/library-1.0.xpl
"/>

    <p:declare-step type="pxf:copy">
        <p:output port="result" primary="false"/>
        <p:option name="href" required="true"/>
        <p:option name="target" required="true"/>
        <p:option name="fail-on-error" select="'true'"/>
    </p:declare-step>

    <p:group>
        <p:variable name="from" select="'image.png'"/>
        <p:variable name="to" select="'image-copy.png'"/>
        <p:variable name="href" select="'image-copy.png'"/>

        <!-- This works -->
        <!--<pxf:copy name="copy">
            <p:with-option name="href" select="$from"/>
            <p:with-option name="target" select="$to"/>
        </pxf:copy>-->

        <!-- This does not work -->
        <pxf:copy name="copy">
            <p:with-option name="href" select="$from"/>
            <p:with-option name="target" select="$href"/>
        </pxf:copy>
    </p:group>

</p:declare-step>
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My ~/.calabash file  contains the following:

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<?xml version="1.0"?>
<xproc-config xmlns="http://xmlcalabash.com/ns/configuration" xmlns:pxf="
http://exproc.org/proposed/steps/file">
    <implementation type="pxf:copy"
class-name="com.xmlcalabash.extensions.fileutils.Copy"/>
</xproc-config>
--------------------------------------------------------------------------------


Bug?

Regards
Jostein