> In XSLT 2.0, choose the first item in a sequence of many 
> items, priority indicated by the order of your sequence:
> 
> <xsl:sort select="
>   ( monogr/author, analytic/author, monogr/editor, 
> monogr/title )[1]"/>
> 
> In XSLT 1.0 you might do something along the lines of:
> 
> <xsl:sort select="
>    monogr/author |
>    self::*[not(monogr/author)]/analytic/author |
>    self::*[not(monogr/author | analytic/author)]/monogr/editor |
>    self::*[not(monogr/author | analytic/author | 
> monogr/editor)]/monogr/title"/>
> 

Would it work to sort on the concatenation? -

<xsl:sort select="concat(monogr/author, analytic/author, monogr/editor,
monogr/title"/>

Michael Kay
http://www.saxonica.com/