xslt 1.0 - find unique values by several criteria - xslt

Question

I have following XML:
<library>
<elements>
<element name="books">
<property name="author">A</property>
<property name="select">true</property>
</element>
<element name="books">
<property name="author">B</property>
<property name="select">false</property>
</element>
<element name="books">
<property name="author">C</property>
<property name="select">true</property>
</element>
<element name="books">
<property name="author">A</property>
<property name="select">true</property>
</element>
</elements>
</library>
I need to get output of all elements with name="books", which are selected (selected = true) and unique by author name.
Must use xslt 1.0.
Expected result:
author: A
author: C
Must output data only for authors A and C.
Thanks in advance!

Answer 1

not(.=preceding::*)
to retrieve the unique values of the given xpath in for loop
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
<xsl:output method="text"/>
<xsl:template match="/">
<ul>
<xsl:for-each select="//elements/element[#name = 'books' and property[#name = 'select' and .='true'] ]/property[#name = 'author' and not(.=preceding::*)]">
<li>
<xsl:value-of select="concat('author :',.)"/>
</li>
</xsl:for-each>
</ul>
</xsl:template>
</xsl:stylesheet>
OUTPUT XML :
author :A author :C