How to show correctly tabulation in xsl-fo?

I have an XML document and I am creating an XSL-FO file to convert it to pdf with apache-fop.

In the xml, there are sections <code> to show... code.

In the xsl-fo, I added the white-space="pre" sentence to preserve the code format, but tabulations are shown like single space:

XML section:

<code><![CDATA[
function callback( widget )
{
    var ui = widget; // It should be a tab
}
]]></code>

XSL-FO section:

<xsl:template match="code">
    <fo:block font-size="10pt" font-family="monospace" white-space="pre" color="#008000" background-color="#f8f8f8">
        <xsl:apply-templates/>
    </fo:block>
</xsl:template>

Resulting PDF:

function callback( widget )
{
 var ui = widget;  // It should be a tab
}

So my question is: How to preserve or set the size of the tabulation?

Edited: I am using apache-fop 1.1

A full example:

proyecto.xml (do not forget to replace the 4 spaces by a tab before "var ui...")

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="proyecto.xsl"?>
<document>
<code><![CDATA[
function callback( widget )
{
    var ui = widget; // It should be a tab
}
]]></code>
</document>

proyecto.xsl

<xsl:stylesheet
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0"
    xmlns:fo="http://www.w3.org/1999/XSL/Format">

    <xsl:template match ="document">
        <fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format">
            <fo:layout-master-set>
                <fo:simple-page-master master-name="OnePage" margin="1in" 
                    page-height="29.7cm" 
                    page-width="21cm"
                    margin-top="2.5cm"
                    margin-bottom="2.5cm"
                    margin-left="3.5cm"
                    margin-right="2.5cm">
                    <fo:region-body margin="0cm"/>
                </fo:simple-page-master>
                <fo:page-sequence-master master-name="Page">
                    <fo:single-page-master-reference master-reference="OnePage"/>
                </fo:page-sequence-master>
            </fo:layout-master-set>

            <fo:page-sequence master-reference="Page">
                <fo:flow flow-name="xsl-region-body">
                    <xsl:apply-templates/>
                </fo:flow>
            </fo:page-sequence>
        </fo:root>
    </xsl:template>

    <xsl:template match="code">
        <fo:block font-size="10pt" font-family="monospace" white-space="pre">
            <xsl:apply-templates/>
            <!--<xsl:value-of select="replace( . , 'a', 'b')"/>-->
        </fo:block>
    </xsl:template>

</xsl:stylesheet>

PDF result (screen):

Replace the tabs by four spaces, as suggested by @mzjn already in the comments.

XML Input

<code><![CDATA[
function callback( widget )
{
&#09;var ui = widget; // It should be a tab
}
]]></code>

XSLT Stylesheet

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="2.0"
      xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
      xmlns:fo="http://www.w3.org/1999/XSL/Format">
  <xsl:output method="xml" indent="yes"/>

  <xsl:template match="/">
    <fo:root>
      <fo:layout-master-set>
        <fo:simple-page-master master-name="A4-portrait"
              page-height="29.7cm" page-width="21.0cm" margin="2cm">
          <fo:region-body/>
        </fo:simple-page-master>
      </fo:layout-master-set>
      <fo:page-sequence master-reference="A4-portrait">
        <fo:flow flow-name="xsl-region-body">
          <fo:block font-size="10pt" font-family="monospace" linefeed-treatment="preserve" white-space-collapse="false" white-space-treatment="preserve" wrap-option="no-wrap" color="#008000" background-color="#f8f8f8">
            <xsl:value-of select="replace(code,'&#09;','    ')"/>
          </fo:block>
        </fo:flow>
      </fo:page-sequence>
    </fo:root>
  </xsl:template>

</xsl:stylesheet>

Now, after replacing the tab characters with whitespaces, the output is rendered correctly by FOP.

There is no obvious flaw with outputting tabs, but bear in mind that Apache FOP is only partially compliant in regard to the whitespace-treatment property: http://xmlgraphics.apache.org/fop/compliance.html , even if it says that only fo:inline elements are affected of this.