Preserving priority of operands in one-liner Xpath 1.0 - xslt

Question

I can't really formulate it properly, better with example.
XML:
<?xml version="1.0" encoding="UTF-8"?>
<foo>
<bar id="someId" class="someClass"/>
<buz class="someClass" id="someId"/>
<ololo class="someClass"/>
<test id="someId"/>
</foo>
XSLT:
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="text"/>
<xsl:strip-space elements="*"/>
<xsl:template match="/*/*">
<xsl:value-of select="#id | #class"/><xsl:text>
</xsl:text>
</xsl:template>
</xsl:stylesheet>
The result:
someId
someClass
someClass
someId
What I need
I need that "priority" of attributes remained as stated in my xpath expression.
So, if we call #id | #class an expression with two operands, I need that the attributes would be taken not in a document order, but in the order of how two operands were specified in the expression.
So, the result should be:
someId
someId
someClass
someId
#class should be taken only if #id is not present.
I know, that it can be done with conditional logic, but I'm really interested in a short solution, because it's common and used as attribute value template.
It might be obvious and I am missing The Elegant One.
Do note that I'm speaking in terms of XPath 1.0.

Answer 1

Use:
#id | #class[not(../#id)]
This XPath expression selects always one node: #id if it exists, and only if #id doesn't exist then #class.
So this transformation:
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:strip-space elements="*"/>
<xsl:output method="text"/>
<xsl:template match="foo/*">
<xsl:value-of select="#id | #class[not(../#id)]"/>
<xsl:text>
</xsl:text>
</xsl:template>
</xsl:stylesheet>
when applied on the provided XML document:
<foo>
<bar id="someId" class="someClass"/>
<buz class="someClass" id="someId"/>
<ololo class="someClass"/>
<test id="someId"/>
</foo>
produces the wanted, correct results:
someId
someId
someClass
someId