Under 18.1.1 in XSLT 2.0 and 23.1.1 in XSLT 3.0, a minor error appears in the following example:

<out xsl:version="2.0">
  <xsl:choose>
    <xsl:when test="function-available('matches')">
      <xsl:value-of select="matches($input, '[a-z]*')"/>
    </xsl:when>
    <xsl:otherwise>
      <xsl:value-of select="string-length(
                          translate($in, 'abcdefghijklmnopqrstuvwxyz', '')) = 0"/>
    </xsl:otherwise>
  </xsl:choose>
</out>

The example assumes a variable $input, but under <xsl:otherwise> the variable is called $in. 

In addition, instead of using an undeclared variable, the example is perhaps clearer and more correct if an XPath axis step were used, like title or username. This also applies to the second example using use-when.