Thanks, Ken!  I should have been clearer in my description.  Each <a1> and
<a2> should be paired, even though there may be intervening tags.  However, an
<a2> without its paired <a1> should not be processed.  I'll try your grouping
solution.

Dave

-----Original Message-----
From: G. Ken Holman [mailto:gkholman@xxxxxxxxxxxxxxxxxxxx]
Sent: Thursday, April 23, 2009 1:26 PM
To: xsl-list@xxxxxxxxxxxxxxxxxxxxxx
Subject: Re:  Need test for preceding-sibling

At 2009-04-23 14:01 -0400, Hintz, David L wrote:
>I have a structure like this:
>
><a1/>
><a2/>
><a1/>
><a2/>
><b/>
><c/>
><a2/>
><a1/>
><a2/>
>
>I need to ensure that each <a2> has a preceding <a1>, although there
>may be intervening tags.

This description is a bit imprecise ... do you need them paired, or
will any <a1> before an <a2> suffice?

I ask because in the example you have every <a2> has a preceding
<a1>, some with intervening tags and some without.

But it isn't true that every <a2> has a preceding <a1> that follows
any preceding <a2>.

>I know that
>
>         name(preceding-sibling::*[1]
>
>gives me the name of the first preceding sibling, but how do I
>construct a test that gives me only the first preceding <a1> or <a2> tag?

You shouldn't be working with name() because that deals with strings
and not expanded node names in a namespace-safe fashion.

The test that there is any <a1> before the current node being <a2> is:

    preceding-sibling::a1

The test that there is an <a1> correspondingly before every <a2> is
approached more easily with XSLT 2 grouping from the parent of those
siblings:

   <xsl:for-each-group select="*"
                       group-ending-with="a2">
     <xsl:if test="not(current-group()/self::a1)">
        Error!  No corresponding a1 for the given a2
     </xsl:if>
   </xsl:for-each-group>

I hope this helps.

. . . . . . . . . . . . Ken

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