(using XSLT 1)
I would like to group numbers by if the following number is current number + 1 and so on.

Example:

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

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

Any insight will be appreciated.

T:\ftemp>type mario.xml
<?xml version="1.0" encoding="UTF-8"?>
<as>
 <a>1</a>
 <a>3</a>
 <a>4</a>
 <a>5</a>
 <a>7</a>
 <a>9</a>
 <a>10</a>
</as>
T:\ftemp>type mario.xsl
<?xml version="1.0" encoding="US-ASCII"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                version="1.0">

<xsl:key name="firsts"
         match="a"
   use="generate-id((. | preceding-sibling::a)
                    [not(. = preceding-sibling::a[1] + 1)]
                    [last()])"/>

<xsl:template match="as">
  <groups>
    <xsl:for-each select="a[key('firsts',generate-id(.))]">
      <group>
        <xsl:copy-of select="key('firsts',generate-id(.))"/>
        </group>
    </xsl:for-each>
  </groups>
</xsl:template>

</xsl:stylesheet>
T:\ftemp>xslt mario.xml mario.xsl
<?xml version="1.0" encoding="utf-8"?>
<groups>
   <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>
</groups>
T:\ftemp>

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