Actually, I first thought about using the "group-adjacent" attribute:

<xsl:stylesheet version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
 <xsl:output omit-xml-declaration="yes" indent="yes"/>
 <xsl:template match="/*">
   <xsl:for-each-group select="a"
                 group-adjacent="number(.) eq
number(following-sibling::a[1]) -1">
    <xsl:if
       test="not(position() eq last)
            or
             not(number(current-group()[last()])
               eq
                 number(current-group()[last() -1]/preceding-sibling::a[1])+1)">
     <group>
       <xsl:sequence select=".[not(number(.) eq
number(preceding-sibling::a[1]) +1)]"/>
       <xsl:sequence select="current-group()[position() gt 1]"/>
       <xsl:sequence select="current-group()[last()]
                                      /following-sibling::a[1]

[number(current-group()[last()]) + 1 eq number(.)]"/>
     </group>
    </xsl:if>
   </xsl:for-each-group>
 </xsl:template>
</xsl:stylesheet>

When this transformation is applied on the provided source XML  document:

<t>
 <a>1</a>
 <a>3</a>
 <a>4</a>
 <a>5</a>
 <a>7</a>
 <a>9</a>
 <a>10</a>
</t>

the wanted, correct result is produced:

<group>
 <a>1</a>
</group>
<group>
 <a>3</a>
 <a>4</a>
 <a>5</a>
</group>
<group>
 <a>7</a>
</group>
<group>
 <a>9</a>
 <a>10</a>
</group>
<group/>

Cheers,
Dimitre

On Mon, Mar 24, 2014 at 8:05 AM, David Carlisle <davidc@xxxxxxxxx> wrote:
> On 24/03/2014 14:44, G. Ken Holman wrote:
>>
>> You can use keys as illustrated below.
>
>
> yeh that completes the triple: for-each-group (DC), sibling recursion
> (MK) and Muenchian grouping (KH). For bonus points we should get DN
> to post a divide and conquer version:-)
>



-- 
Cheers,
Dimitre Novatchev
---------------------------------------
Truly great madness cannot be achieved without significant intelligence.
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To invent, you need a good imagination and a pile of junk
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To avoid situations in which you might make mistakes may be the
biggest mistake of all
------------------------------------
Quality means doing it right when no one is looking.
-------------------------------------
You've achieved success in your field when you don't know whether what
you're doing is work or play
-------------------------------------
To achieve the impossible dream, try going to sleep.
-------------------------------------
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-------------------------------------
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I finally figured out the only reason to be alive is to enjoy it.