Description : Normally Xsl layouts are rendered in the following order:

 1)      Calculating input parameters
 2)      Execution of XSLT transform
 3)      Execution of the function calls that are left after the transformation

The "CallFunction" extension method allows to use a result of a function call during the xsl transform.
Example of using:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0"
 .....
 xmlns:c1="http://c1.composite.net/StandardFunctions">

 <xsl:template match="/">
     ...............
     <!-- Declaring a function call markup in a variable -->
     <xsl:variable name="SendMailFunctionMarkup">
      <f:function xmlns:f="http://www.composite.net/ns/function/1.0" name="Composite.Mail.SendMail">
       <f:param name="From" value="{$from}" />
       <f:param name="To" value="{$to}" />
          ...
      </f:function>
     </xsl:variable>

     <!-- Calling "SendMail" function, and saving a result (boolean in the current case) to a variable -->
     <xsl:varibale name="emailSentSuccessfully" select="c1:CallFunction($SendMailFunctionMarkup)" />
    ....
 </xsl:template>
</xsl:stylesheet>