code:
  
<?xml version="1.0"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
	<xsl:template match="/">
		<root>
			<xsl:apply-templates select="*/left[1]"/>
		</root>
	</xsl:template>
	<xsl:template match="left">
		<xsl:choose>
			<xsl:when test="following::left">
				<xxx>
					<xsl:copy-of select="following::*[count(following::left) = count(current()/following::left)]"/>
					<xsl:apply-templates select="following::left[1]"/>
					<xsl:apply-templates select="following::right[last()- count(current()/preceding::left)]"/>
				</xxx>
			</xsl:when>
			<xsl:otherwise>
				<xxx>
					<xsl:copy-of select="following::*[count(following::right) = count(current()/following::right)]"/>
				</xxx>
			</xsl:otherwise>
		</xsl:choose>
	</xsl:template>
	<xsl:template match="right">
		<xsl:copy-of select="preceding::*[count(preceding::right) = count(current()/preceding::right)]"/>
	</xsl:template>
</xsl:stylesheet>