Note that with XSLT 2.0 there is almost never a need to use document('') and
it should generally (in my view) be avoided. Even with 1.0, it is often used
to read "lookup data", and in my view it's better to put such lookup data in
a separate document rather than putting it in the stylesheet.

<xsl:template name="ra:sayHello" match="xsl:template[@name='ra:sayHello']">
       <xsl:text>Hello!</xsl:text>
</xsl:template>
...
<xsl:variable name="n" select='ra:sayHello'/>
<xsl:apply-templates select="document('')/*/xsl:template[@name=$n]">