I am trying to access 'thing' elements by the value of their 'a'
element children.
I do not know what the contents of the 'a' elements will be, so I
gather the unique values and iterate through them. When I iterate
through, I can print the value, placing the value into the key does
not return the 'things' that have 'a' children with that value.
Additionally, placing a literal string will access the desired nodes,
but a variable with the same string content does not.

The input, current output, desired output, xslt (with line numbers)
and xslt are listed below.

Many thanks,
Tim

======= Input ========
<things>
 <thing>
   <name>thing_1</name>
   <a>one</a>
 </thing>
 <thing>
   <name>thing_3</name>
   <a>one</a>
   <a>two</a>
 </thing>
 <thing>
   <name>thing_4</name>
   <a>one</a>
   <a>two</a>
   <a>three</a>
 </thing>
 <thing>
   <name>thing_5</name>
   <a>three</a>
 </thing>
</things>

======= Current Output =========
Dynamic Selection (DOES NOT WORK)
one 0
two 0
three 0

======= Desired output ========
Hard-coded selection (DESIRED OUTPUT)
one 3
two 2
three 2

======== XSLT =========
     1 <xsl:transform version="2.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
     2 <xsl:output method="text"/>
     3 <xsl:key name="things-by-a" match="thing" use="a"/>
     4
     5 <xsl:template match="/">
     6   <xsl:variable name="as">
     7     <xsl:for-each-group select="//a" group-by=".">
     8       <a><xsl:value-of select="current-grouping-key()"/></a>
     9     </xsl:for-each-group>
    10   </xsl:variable>
    11
    12   <xsl:value-of select="concat($NL,'Dynamic Selection (DOES
NOT WORK)',$NL)"/>
    13   <xsl:for-each select="$as/a">
    14     <xsl:value-of select="concat(.,'
',count(key('things-by-a',.)),$NL)"/>
    15   </xsl:for-each>
    16
    17   <xsl:value-of select="concat($NL,'Hard-coded selection
(DESIRED OUTPUT)',$NL)"/>
    18   <xsl:value-of select="concat('one
',count(key('things-by-a','one')),$NL)"/>
    19   <xsl:value-of select="concat('two
',count(key('things-by-a','two')),$NL)"/>
    20   <xsl:value-of select="concat('three
',count(key('things-by-a','three')),$NL)"/>
    21 </xsl:template>
    22
    23 <xsl:variable name="NL">
    24 <xsl:text>
    25 </xsl:text>
    26 </xsl:variable>
    27
    28 </xsl:transform>

======== XSLT w/o line numbers =========
<xsl:transform version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="text"/>
<xsl:key name="things-by-a" match="thing" use="a"/>

<xsl:template match="/">
 <xsl:variable name="as">
   <xsl:for-each-group select="//a" group-by=".">
     <a><xsl:value-of select="current-grouping-key()"/></a>
   </xsl:for-each-group>
 </xsl:variable>

 <xsl:value-of select="concat($NL,'Dynamic Selection (DOES NOT WORK)',$NL)"/>
 <xsl:for-each select="$as/a">
   <xsl:value-of select="concat(.,' ',count(key('things-by-a',.)),$NL)"/>
 </xsl:for-each>

 <xsl:value-of select="concat($NL,'Hard-coded selection (DESIRED
OUTPUT)',$NL)"/>
 <xsl:value-of select="concat('one ',count(key('things-by-a','one')),$NL)"/>
 <xsl:value-of select="concat('two ',count(key('things-by-a','two')),$NL)"/>
 <xsl:value-of select="concat('three
',count(key('things-by-a','three')),$NL)"/>
</xsl:template>

<xsl:variable name="NL">
<xsl:text>
</xsl:text>
</xsl:variable>