Input xml contains
<data>
...
    <membership>....</membership>
</data>
where membership can be one of a b c d

cmd line param
membership=a
could be membership=a,b,c

How to 'choose' only those matching the param values?
<xsl:template match="data">
<xsl:if test="$membership =./membership">
     <xsl:message>
    <xsl:value-of select='membership_type'/>
  </xsl:message>
     <xsl:apply-templates/>
   </xsl:if>
..

How to pass a list, and then check for membership
of that list, one or more times?

or use an XPath expression for the parameter with e.g. (Saxon 9/10/11, I
think)