How to perform a two Part XSLT

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How would I change this\[code\]<CHAPT> <CHAPTNO>1.1</CHAPTNO> <SUBJ>Introduction</SUBJ> <P>foo text</P> <P>foo text</P> .. and so on</CHAPT>\[/code\]To this:\[code\]<p class="chapheader"><span class="chapnumbers">1.1 </span>Introduction</p><P>foo text</P><P>foo text</P>\[/code\]But, only where \[code\]<CHAPTNO>\[/code\] directly precedes \[code\]<SUBJ>\[/code\] ..because there are other \[code\]<SUBJ>\[/code\] in the XML that do not follow a \[code\]<CHAPTNO>\[/code\] and I do not want those stand alone Subject Lines formatted as a Chapter header.I'm new to XSLT and got stuck on my second template.