Adding checkbox to XForms repeat structure

I'm using XSLTForms in eXist 2.2 with the RESTXQ server.

I have a search form that lets users query a remote API, which responds with a set of XML records if the query is matched. I am using xf:repeat to iterate over the records, and I want to be able to provide a checkbox at the head of each record, so that the user can choose the records they want. However, when I place a checkbox element inside xf:repeat (using xf:input bound to a boolean value), I don't get the desired functionality. Instead of being independent of each other, the checkboxes get activated as a group. When I click the first box, the second box is activated as well, etc. This seems as though it would be a common enough use case, but I can't seem to find any documentation about how to implement it.

Model snippet:

<xf:instance xmlns="" id="default">
    <results>
        <sru:searchRetrieveResponse>
            <sru:records>
                <sru:record>
                ...
                </sru:record>
            </sru:records>
        </sru:searchRetrieveResponse>
        <sru:searchRetrieveResponse>
            <sru:records>
                <sru:record>
                ...
                </sru:record>
            </sru:records>
        </sru:searchRetrieveResponse>
    </results>
</xf:instance>

<xf:instance xmlns="" id="test">
    <data>   
        <bool>false</off>                                     
    </data>
</xf:instance>

<xf:bind nodeset="instance('test')/bool" id="boolVal" type="xs:boolean"/>

Form snippet with xf:repeat:

<div>
    <xf:repeat 
    nodeset="instance('default')/sru:searchRetrieveResponse/sru:records/sru:record" 
    id="marc-repeat" appearance="full">
        <div class="checkbox">
            <xf:input incremental="true" ref="instance('test')/bool">
                <xf:label>Select</xf:label>                                 
            </xf:input>
        </div>
        ...
    </xf:repeat>
</div>