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[xsl] curious behavior of select= and predicates

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  [more pedantics involving simple examples.  the following results
are from "xsltproc", so it may be a processor-related thing.]

cars.xml
--------

<?xml version="1.0"?>

<cars>
 <car>
  <model>mazda mx-6</model>
  <displacement>123</displacement>
 </car>

 <car>
  <model>yugo</model>
 </car>

 <car>
  <model>vette</model>
  <displacement>423</displacement>
 </car>

 <car>
  <model>lamborghini</model>
 </car>

</cars>

dumb.xsl (or at least the only important part of it)
----------------------------------------------------

<xsl:template match="cars">
 <xsl:apply-templates select="car"/>
</xsl:template>

<xsl:template match="car">
 <xsl:value-of select="self::node()[displacement]"/>
</xsl:template>

</xsl:stylesheet>


First puzzler
-------------

  With dumb.xsl as above, i'm using the "car" template to 
decide itself what to return as a result of <xsl:value-of> --
the string-value of the node only if there is a displacement child
element.  *that* works fine (even if it is a silly way to do it.)

  First oddity -- i had always understood that "self::node()" could
be abbreviated as just ".".  but if i replace the select expression
with ".[displacement]", i get the error

================
XPath error Invalid expression
.[displacement]
 ^
compilation error: file dumb.xsl line 13 element value-of
xsl:value-of : could not compile select expression '.[displacement]'
=================

  should i not expect "self::node()[displacement]" to be logistically
equivalent to ".[displacement]"?  or did i misread something?
  
Second puzzler
--------------

  I want the "car" template to return its string value only if it's
the third in context position.  (again, a weird thing to do but humor
me.)  shouldn't i be able to write:

<xsl:template match="car">
 <xsl:value-of select="self::node()[position() = 3]"/>  # just pos 3
</xsl:template>

  instead, i get nothing, even though i've verified that the consecutive
invocations of the "car" template do, in fact, run through the context
positions 1 -> 4 for each "car" element.

  again, have i misread how i can refer to the context node itself
in a select expression?

rday


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