Numbering non-contiguous nodes

Hello all,

I'm looking for the simplest and most elegant solution to a node 
numbering problem. Below is an example of the relevant part of my XML 
code:

===========
XML SNIPPET
*****************

<question>
The 
<gap>
        <option>
                <value>blue whale</value>
                <feedback>Are whales really fish?</feedback>
        </option>
        <option>
                <value>giant squid</value>
                <feedback>Have you ever seen a fish with ten arms?</feedback>
        </option>
        <option>
                <value>whale shark</value>
                <feedback>Congratulations!</fee dback>
        </option>
        <alt_feedback>Make sure you think of a FISH.</alt_feedback>
</gap>
is the largest fish on Earth. It feeds mainly on  
<gap>
        <option>
                <value>human flesh</value>
                <feedback>Does the text say that all sharks are 
aggressive?</feedback>
        </option>
        <option>
                <value>fish</value>
                <feedback>Are shrimp a kind of fish?</feedback>
        </option>
        <option>
                <value>krill</value>
                <feedback>All right!</feedback>
        </option>
        <alt_feedback>Which other animal are whale sharks compared 
to?</alt_feedback>
</gap>
.
</question>

===========

As you can see, each "question" element may contain text and any 
number of "gap" elements. Now, basically, I need my output to be like 
this:

===============
DESIRED OUTPUT
**********************

<p>The [_1_] is the largest fish on Earth. It feeds mainly on  
[_2_].</p>

===============
In other words, all text nodes should appear in the output, but the 
"gap" element nodes should be replaced with "[_" + gap number + "_]", 
where "gap number" is the sequential number of each "gap" element.

I thought of using XSLT code along these lines:

==============
PROPOSED XSLT
*********************

<xsl:template match="question">
        <xsl:for-each select="node()">
                <xsl:choose>
                        <xsl:when test="local-name() = 'gap'">
                                <xsl:text>[_</xsl:text>
                                <xsl:value-of select="position()"/>
                                <xsl:text>_]</xsl:text>
                        </xsl:when>
                        <xsl:otherwise>
                                <xsl:value-of select="."/>
                        </xsl:otherwise>
                </xsl:choose>
        </xsl:for-each>
</xsl:template>

==============

The problem is that I only want the "gap" nodes to be counted, but 
the above solution counts all child nodes of "question" (including 
the text nodes), so the numbering is incorrect for my purposes. I 
thought of using a variable as a counter that would be incremented 
each time <xsl:when test="local-name() = 'gap'"> evaluated to true, 
but after considerable research have come to the [possibly 
inaccurate] conclusion that XSL variables cannot be reassigned values 
this way.

Any and all suggestions will be greatly appreciated.

Erik

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