'Variable' question from Newbie

I (superficially) understand that you can't change the value of a variable
once it's defined.  But although I've looked through the list FAQs and other
sources, I haven't yet grasped how one goes about doing certain things that
would be possible, if only one could update a variable each time through a
loop.  Here's my problem.

I have a collection of publications records in XML that store Discipline,
Title, Date, and other characteristics, including a flag indicating whether
the publication should show up on a Highlights page.  I now want to output
the Highlights page, and I want the value of Discipline to be output only
once per group of records sharing that discipline.  E.g. 

JAILS
Title of Report1, publisher1, date1
Title of Report2, publisher2, date2
[link to Jails subpage]

PRISONS
Title of Report3, publisher3, date3
Title of Report4, publisher4, date4
[link to Prisons subpage]

In the olden days I would have used something like the following logic:

Sort by Discipline
For each record,
Check if Highlights = True
If yes,
  Check if Discipline = $var
  If no,
    Put the value of Disciplines into $var
    Output the value of $var
    Output values of Title, Publisher, and Date
  If yes,
    Output values of Title, Publisher, and Date


I can't do it like this in XSLT.  In fact, I have to learn not to THINK like
this, I'm starting to realize.  Can someone please explain to me how I would
accomplish the same end?  Thanks very much in advance for any help.  Below
is a sample of my XML.

~Stacy


<InstCorr>
	<Record>
		<Title>BJS Drugs &amp; Crime Facts</Title>
		<Disciplines>Institutional Corrections, Jails</Disciplines>
		<Highlights_Discipline>1</Highlights_Discipline>
	</Record>
	<Record>
		<Title>Drug Treatment in the Criminal Justice System</Title>
		<PubDate>March 2001</PubDate>
		<Disciplines>Institutional Corrections, Jails</Disciplines>
		<Highlights_Discipline>0</Highlights_Discipline>
	</Record>
	<Record>
		<Title>Assessing Alcohol, Drug, and Mental Disorders in
Juvenile Detainees</Title>
		<PubDate>January 2001</PubDate>
		<Disciplines>Institutional Corrections, Jails</Disciplines>
		<Highlights_Discipline>1</Highlights_Discipline>
		</Record>
	<Record>
		<Title>TRIAD Drug Treatment Evaluation Project</Title>
		<PubDate>September 2000</PubDate>
		<Disciplines>Institutional Corrections,
Prisons</Disciplines>
		<Highlights_Discipline>0</Highlights_Discipline>
	</Record>
	<Record>
		<Title>Institute for Criminal Justice Healthcare</Title>
		<Disciplines>Institutional Corrections,
Prisons</Disciplines>
		<Highlights_Discipline>1</Highlights_Discipline>
	</Record>
	<Record>
		<Title>Therapeutic Communities in Correctional
Settings</Title>
		<PubDate>November 1999</PubDate>
		<Disciplines>Institutional Corrections,
Prisons</Disciplines>
		<Highlights_Discipline>1</Highlights_Discipline>
	</Record>
	<Record>
		<Title>Substance Abuse Treatment Programs in the Federal
Bureau of Prisonss</Title>
		<PubDate>January 1999</PubDate>
		<Disciplines>Institutional Corrections,
Prisons</Disciplines>
		<Highlights_Discipline>0</Highlights_Discipline>
	</Record>
</InstCorr>





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