Enclosing sequence items in single quotes

I need some help in constructing a string from a sequence. Consider this template:

<xsl:template match="/">
    <xsl:variable name='pf-codes'><xsl:value-of select="position-translation/position/code[@pf='pf']"/></xsl:variable>
    <xsl:variable name="pf-codes-string" select="for $s in $pf-codes return $s"/>
     <xsl:value-of select="$pf-codes-string"/>
</xsl:template>

This produces output like this:

AA BB CC DD EE FF GG ...

I need output like this:

'AA','BB','CC','DD','EE','FF','GG', ..., 'ZZ'

Note that the last item is not followed by a comma.

I haven't moved on to the question of using position()=last() to bring about this last requirement.

I'm having trouble constructing an expression that will enclose each item in the sequence in quotes (single).

Please point me in the right direction. Thanks

-- 
Charles Knell
cknell@xxxxxxxxxx - email