To achieve the same functionality with XSLT and XPath, we would do this:

<xsl:variable name="isDifferent" select="$FIRST ne $SECOND"

<xsl:variable name="APPEND" as="xs:string?">
     <xsl:if test="$isDifferent eq true()">
         <xsl:sequence select="concat($FIRST, $SECOND)"/>
     </xsl:if>
</xsl:variable>

B  <xsl:variable name="APPEND" as="xs:string?" select="($FIRST ||
$SECOND)[$isDifferent]"/>