XPath based context and filtering for submission?

Itwouldbeniceif there were a way to filter a list of matching nodes to
submit.

I would like to submit only the index numbers of items whose @selected
attribute is true.

The present definition of submission/@ref uses the first nodeset rule,
so you cannot just do
  <submission
ref="instance('data')/lines/line[boolean-from-string(@selected)]/index"
/>
Additionally, the ref attribute doesn't appear to be precise enough to
specify what parts of the tree to submit and what parts to leave out.

Some simple XPath-based filtering syntax like that would be attractive,
though.

Here's one way to do it, using a control variable (set during submission
only) and a pair of bind elements, one to say the
"/lines/line[boolean-from-string(@selected)]" part and another to say
that "index" part. 

Can anybody think of a clever union expression to put in submission/@ref
that just does what I want?

<instance id="data">
  <data xmlns="">
    <lines>
      <line selected="false">
        <index>1</index>
        <date>2006-01-01T00:00:00Z</date>
        <name>A</name>
        <text>...</text>
      </line>
      <line selected="false">
        <index>2</index>
        <date>2006-01-01T01:00:00Z</date>
        <name>B</name>
        <text>...</text>
      </line>
      <line selected="false">
        <index>3</index>
        <date>2006-01-01T02:00:00Z</date>
        <name>C</name>
        <text>...</text>
      </line>
     </lines>
  </data>
</instance>

<instance id="results">
  <empty xmlns=""/>
</instance>

<instance id="control">
  <control xmlns="">
    <selected>false</selected>
  </control>
</instance>

<bind nodeset="instance('data')lines/line/@selected" type="xsd:boolean"
/>
<bind nodeset="instance('control')/selected" type="xsd:boolean" />

<bind nodeset="instance('data')/lines/*[local-name(.)!='index']" 
      relevant="not(boolean-from-string(instance('control')/selected))
<bind nodeset="instance('data')/lines/line"
	relevant="not(boolean-from-string(instance('control')/selected))
or boolean-from-string(@selected)"/>

<submission id="delete" action="/delete" ref="instance('data')"
replace="instance" instance="results">
   <setvalue ev:event="xforms-submit"
ref="instance('control')/selected">true</setvalue>
   <setvalue ev:event="xforms-submit-done"
ref="instance('control')/selected">false</setvalue>
   <setvalue ev:event="xforms-submit-error"
ref="instance('control')/selected">false</setvalue>
</submission>

<submit submission="delete">
  <label>Delete</label>
</submit>

<repeat id="lines" nodeset="lines/line">
  <input ref="@selected">
    <label class="columnHeader">X</label>
  </input>
  <output ref="date">
    <label class="columnHeader">Date</label>
  </output>
  <output ref="name">
    <label class="columnHeader">Name</label>
  </output>
</repeat>

<output ref="lines/line[index('lines')]/text">
  <label>Text</label>
</output>

Received on Friday, 1 December 2006 00:00:30 UTC