I have following input XML:

  <root>
    <a id="1"/>
    <a id="2"/>
    <b/>
    <d/>
    <g/>
    <a id="3"/>
    <a id="4"/>
    <a id="5"/>
    <x/>
    <a id="6"/>
    <a id="7"/>
  </root>

and I'm trying to create XSLT 1.0 script, which would nest "uninterrupted" sibling groups of 'a' elements into 'a-block' elements

<xsl:stylesheet
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  version="1.0">

<xsl:template match="root">
  <xsl:copy>
    <xsl:apply-templates select="*"/>
  </xsl:copy>
</xsl:template>

<xsl:template match="a" mode="copy">
  <xsl:copy>
    <xsl:apply-templates select="@* | node()"/>
  </xsl:copy>
</xsl:template>

<xsl:template match="* | @*">
  <xsl:copy>
    <xsl:apply-templates select="@* | node()"/>
  </xsl:copy>
</xsl:template>