Input:
<p>These pages are all about XSLT, an XML-based language <xref ref-type="bibr" rid="r1">1</xref><xref ref-type="bibr" rid="r2"/>--<xref ref-type="bibr" rid="r3">3</xref> for translating one set of XML into another set of XML, <xref ref-type="bibr" rid="r3">3</xref>, <xref ref-type="bibr" rid="r5">5</xref><xref ref-type="bibr" rid="r6"/>--<xref ref-type="bibr" rid="r7">7</xref> or into HTML. Of course, there are all sorts of other pages  <xref ref-type="bibr" rid="r1">7</xref>, <xref ref-type="bibr" rid="r3">8</xref>  around that cover XSLT. <xref ref-type="bibr" rid="r12">12</xref>, <xref ref-type="bibr" rid="r15">15</xref><xref ref-type="bibr" rid="r16"/><xref ref-type="bibr" rid="r17"/><xref ref-type="bibr" rid="r18"/><xref ref-type="bibr" rid="r19"/>--<xref ref-type="bibr" rid="r20">20</xref></p>

output:
<p>These pages are all about XSLT, an XML-based language <xref ref-type="bibr" rid="r1 r2 r3" post-process="true">1--3</xref> for translating one set of XML into another set of XML, <xref ref-type="bibr" rid="r3 r5 r6 r7">3, 5--7</xref> or into HTML. Of course, there are all sorts of other pages <xref ref-type="bibr" rid="r7 r8">7, 8</xref> around that cover XSLT. <xref ref-type="bibr" rid="r12 r15 r16 r17 r18 r19 r20">12, 15--20</xref></p>


The merge should happen only if characters (, )comma with space or ( ) space or (--) two hyphens or empty xref element (<xref ref-type="bibr" rid="r2"/>) appear inbetween xref elements.


E.g.
Input content: <xref ref-type="bibr" rid="r3">3</xref>, <xref ref-type="bibr" rid="r5">5</xref><xref ref-type="bibr" rid="r6"/>--<xref ref-type="bibr" rid="r7">7</xref>  

Expected ouput: <xref ref-type="bibr" rid="r1 r2 r3">1--3</xref>