[xsl:fo] : spaces underlining issues.

[Gilles Kuhn]

I'm using fop and I would like to underline spaces, even those located
out of the character string. To illustrate the problem, let's have a
look at this:

xml doc:
<page>
<line>(<u> abc </u>)</line>
</page>

I would like the result of xsl:fo transformation to be:
( abc )
^^^^^^^^^

but actually, I get this:
( abc )
^^^

with the following code xsl/fo:
.....
<xsl:template match="page">
<xsl:apply-templates select="line"/>
</xsl:template>

<xsl:template match="line">
<fo:block><xsl:apply-templates/></fo:block>
</xsl:template>

<xsl:template match="u">
<fo:inline white-space-collapse="false"
text-decoration="underline"><xsl:value-of select="."/></fo:inline>
</xsl:template>

even the 'white-space-collapse' attribute doesn't help!
Does anyone have an idea?
Thanks a lot in advance.