[xsl] Inserting commas between a list of distinct values and a period (full stop) after the last task

["Mark" <mark@xxxxxxxxxxxx>]

I collect from an XML document a list of the distinct tasks performed during 
multiple sessions with:
<fo:block>
     <xsl:text>Projects Tasked: </xsl:text>
     <xsl:value-of select="distinct-values(//Session/@task)"/>
</fo:block>
This produces a correct list of all the distinct tasks from multiple 
occurrences of the same tasks:
LS Authorities Maintenance Administration Meeting

Which I want to put in the PDF document punctuated as:
LS Authorities, Maintenance, Administration, Meeting.
where ("LS Authorities" is recorded as a single task despite its internal 
space).

Can this be done with the information given?
Thanks,
Mark