Access to string/variables per character

Hello,

I'm trying to linearize MathML equations into a string for blind people to
be able to read it. Here's an exctract of my stylesheet :
[...]
<xsl:template match="math">
	<B>Equation : </B>
        <P>
	<xsl:variable name="copy99">
            <xsl:apply-templates select="@*|node()"/>
	</xsl:variable>
	<xsl:value-of select="$copy99"/>
	</P>
</xsl:template>
[...]

the variable copy99 contains the "raw" linearized MathML equation, and I'd
like to remove or add some characters inside this string : spaces, ( and
)... So there must be a possibility with <xsl:for-each , concat,
substring-before and after, but I can't find any example or explanation
dealing with it... is xsl:for-each only for elements ? How do I access
each char of copy99 ? copy99[1], copy99[2]... ?

Thanks in advance for your reply
Frederic SCHWEBEL


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