Not sure what is use of our Xslt Graph exercises but what we are sure with is that it stresses different parts of Saxon Xslt engine and helps to find and resolve different bugs.

While implementing biconnected components algorithm we incidently run into internal error with Saxon 10.1 with rather simple xslt:

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="3.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:array="http://www.w3.org/2005/xpath-functions/array"
  exclude-result-prefixes="xs array">

  <xsl:template match="/">
    <xsl:sequence select="
      array:fold-left
      (
        [8, 9], 
        (), 
        function($first as item(), $second as item()) 
        {  
          min(($first, $second))
        }
      )"/>
  </xsl:template>

</xsl:stylesheet>

More detail can be found at Saxon's issue tracker: Bug #4578: NullPointerException when array:fold-left|right $zero argument is an empty sequence.

Bug is promptly resolved.