I need to convert a flat structure like this

<root>
    <H>1</H>
    <I>1-1</I>
    <I>1-2</I>
    <I>1-3</I>
    <H>2</H>
    <I>2-1</I>
    <I>2-2</I>
  </root>

in one like this

<root>
    <H>
       1
       <I>1-1</I>
       <I>1-2</I>
       <I>1-3</I>
    </H>
    <H>
       2
       <I>2-1</I>
       <I>2-2</I>
    </H>
</root>

  <xsl:template match="root">
    <xsl:copy>
      <xsl:for-each-group select="*" group-starting-with="H">
        <xsl:copy>
          <xsl:apply-templates select="node(), current-group() except ."/>

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

<xsl:template match="H">
  <xsl:copy>
    <xsl:apply-templates select="node() | key('group', generate-id())"/>
  </xsl:copy>
</xsl:template>