<xsl:function name="wl:trdif">
  <xsl:param name="a" as="xsd:string"/>
  <xsl:param name="b" as="xsd:string"/>
  <xsl:choose>
    <xsl:when test="string-length($a) = 0 or
                    substring($a,1,1) != substring($b,1,1)">
      <xsl:value-of select="$b"/>
    </xsl:when>
    <xsl:otherwise>
      <xsl:value-of select="wl:trdif(substring($a,2),substring($b,2))"/>
    </xsl:otherwise>
  </xsl:choose>
</xsl:function>

Thanks Laun for suggestion, I will look into that.