I've got an XML document with multiple entries of <element> as shown
below. The <name>s are unique, but the <number> can be same for
mulpitle entries of <element>. The XML document isn't sorted in anyway.
...
Now I need the XSL stylesheet to print out something like this:

number 1
        name1
        name4
        name7

number 25
        name2
        name3

number 100
        name5
        name6

So each "number X" is only printed once and then all the <element>s
that have the number X in the <number> tags are printed below that.
Then the next biggest number is printed and so on.

Here's one way to do it:
...
      <xsl:for-each select="data/element[not(number = following::number)]">

Note that using axes is the slowest of the three
methods of doing grouping:  axes, variables and
keys ... but that's not the objective of my post.

<xsl:sort select="number"/>

The problem with the above is that the default
data type is "text", so when I put Lassi's test
data into Jay's structure, I get this incorrect result:

T:\ftemp>xslt jay.xml jay.xsl con
<?xml version="1.0" encoding="utf-8"?>
<out>
   <entry number="1">
      <name>Oscar</name>
      <name>Mike</name>
   </entry>
   <entry number="100">
      <name>Tom</name>
      <name>Dick</name>
      <name>Harry</name>
   </entry>
   <entry number="25">
      <name>Bob</name>
      <name>George</name>
   </entry>
</out>
T:\ftemp>

in order for the values to not be treated as
strings.  I have this in one of my students'
exercises (hockey standings) in order to trip them up in their reporting.

So a complete solution, not using axes and not
needing to have a variable would be as posted
below.  On large data sets this would work very much faster than using axes.

<data>
  <element>
    <name>Oscar</name>
    <number>1</number>
  </element>
  <element>
    <name>Mike</name>
    <number>1</number>
  </element>
  <element>
    <name>Bob</name>
    <number>25</number>
  </element>
  <element>
    <name>George</name>
    <number>25</number>
  </element>
  <element>
    <name>Tom</name>
    <number>100</number>
  </element>
  <element>
    <name>Dick</name>
    <number>100</number>
  </element>
  <element>
    <name>Harry</name>
    <number>100</number>
  </element>
</data>

T:\ftemp>type jay.xsl
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

  <xsl:template match="/">
    <out>
      <xsl:for-each select="data/element[generate-id(.)=
                                         generate-id(key('nbr',number)[1])]"

        <xsl:sort select="number" data-type="number"/>
        <entry number="{number}">
          <xsl:copy-of select="key('nbr',number)/name"/>
        </entry>
      </xsl:for-each>
    </out>
  </xsl:template>

T:\ftemp>xslt jay.xml jay.xsl con
<?xml version="1.0" encoding="utf-8"?>
<out>
   <entry number="1">
      <name>Oscar</name>
      <name>Mike</name>
   </entry>
   <entry number="25">
      <name>Bob</name>
      <name>George</name>
   </entry>
   <entry number="100">
      <name>Tom</name>
      <name>Dick</name>
      <name>Harry</name>
   </entry>
</out>
T:\ftemp>

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