About <xsl:param> and <xsl:with-param>

Hi..

In the next xsl I want to make that the param "cod" be incremented in each time that the <xsl:for-each> cicle occurs, but it's result is an error:

"javax.xml.transform.TransformerException: xsl:with-param is not allowed in this position in the stylesheet!"

Why?!
Tks

<?xml version="1.0" encoding="ISO-8859-1"?>
<xsl:transform xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    version="1.0"
    xmlns:xalan="http://xml.apache.org/xslt">
	<xsl:param name="cod">0</xsl:param>
	<xsl:output method="html"/>
	<xsl:template match="/">
    		<xsl:for-each select=".//Boy">
    			<xsl:with-param name="cod" select="number($cod+1)"/>
			<!--make other things-->
		</xsl:for-each>
	</xsl:template>
</xsl:transform>
    

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