Params and Variables must be at the top of a template?

I'd thought that params and variables must be at the top of a template.. yet i can successfully compile this code:

   <xsl:template name="WriteOptions">
      <xsl:param name="input"/>
      <xsl:choose>
      <xsl:when test="true()">
      </xsl:when>
      <xsl:otherwise>
         <xsl:param name="head" select="substring-before($input, ',')"/>
         <xsl:variable name="tail" select="substring-after($input, ',')"/>
         <option value="{$head}"><xsl:value-of select="$head"/><开发者_JS百科;/option>
      </xsl:otherwise>
      </xsl:choose>
   </xsl:template>

So i was wondering what's the catch

Per the spec, you can write <xsl:param> as a child of <xsl:template> and <xsl:stylesheet> (or <xsl:transform>, which is the alternative top level element in XSLT).

If your stylesheet compiles, then your XSLT processor behaves non-standard.

Only an xsl:param within an xsl:template must be a child of the xsl:template instruction.

There is no such requirement for an xsl:variable -- the W3C XSLT 1.0 and XSLT 2.0 specifications allow them to appear at any depth in the XSLT stylesheet.