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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