I wrote:

> <xsl:template match="sub_a|sub_b|sub_c">
>   <xsl:variable name="copy"
> select="boolean(*/following-sibling::*[local-name() =
> preceding-sibling::*/local-name()])"/>
>
>   <xsl:if test="$copy">
>     <xsl:copy-of select="."/>
>   </xsl:if>
> </xsl:template>

...which relies on the slash operator in 2.0.

A 1.0 solution that I've cobbled together quickly is:

<xsl:template match="sub_a|sub_b|sub_c">
	<xsl:variable name="copy">
		<xsl:for-each select="*">
			<xsl:if test="following-sibling::*[local-name()
= local-name(current())]">true</xsl:if>
		</xsl:for-each>
	</xsl:variable>
	<xsl:if test="contains($copy, 'true')">
		<xsl:copy-of select="."/>
	</xsl:if>
</xsl:template>

(Not very good)